@San_Francisco Thanks a lot, very detailed answer, I’ll do exactly that.
Regarding crypto, no we don’t even own a single one, never tried. We really have an extremely simple situation : two jobs, low expenses, some VT on IBKR, no real estate, no crypto, no side gig. I’ve not even sold a single share, so our stocks are really just buy&forget.
Just that since I used to spend a lot, my savings were basically non existent before, so indeed last year they went up of 100k, I.e. more than doubled.
Oh, what is the rational behind that? Beucase then someone can then hide their money in switzerland not abroad and that’s it… I though that both are shared.
I also expected the total amount received/expend in each account was reported but is it only the balances??
Swiss people still very much care about banking secrecy, so nothing is shared domestically.
But OECD countries care about tax dodging as a whole, and Switzerland doesn’t want to have sanction due to facilitating tax evasion, so Switzerland sends data about foreigners’ bank accounts.
And then because reciprocity is a core principle when it comes to diplomacy/relationship between countries, the data sharing is symmetrical (so because Switzerland is sending data, they’re also receiving data).
PS: I was going by the information you provided here.
Note that I’m not advocating to include as little evidence or information as possible - or for no spreadsheet (or report) under any circumstances.
Timing of ETF purchases during the year can impact your return/year-end change of wealth significantly.
If you have a number of VT shares at the beginning of the year and two or three purchase transactions over the course of the year, figuring out your (realised or unrealised) gain is easy.
If, on the other hand, you have a dozen securities funds with many trades each over the course of the year (that significantly impact your wealth), I, as your taxman, probably don’t want to figure your total out myself from random transaction reports or something that you throw at me in a low-effort way.
Ideally, I suppose, you’d explain something like this:
If that adds up to most of the gain in wealth (they won’t be bothering about hundreds. They may about a couple thousand) and leaves a reasonable spending / expenses for living, what more could they want?
I would expect that they can calculate correctly the capital gains themselves, since everything was declared.
I think that you were able to save much more than other couples with comparable income do and then they suspected undeclared incore or inheritance.
I bet that’s it, esp. since they mentioned the spending was much lower than previous year (so increased income + less spending raises some flag, most people would have lifestyle inflation instead, so good job 
)
So, have you actually tried that? Or is it really that bad over there?
I’ve had the same issue 3 years ago and this year again, as recently as this morning (Geneva).
First time was due to large variation year to year mainly due to crypto assets generating large capital variation during the year. As those are declared as cash account (there’s no other way to declare them) with simply the CHF value as reference, they couldn’t really track the reason for the difference in value as there’s no asset unit declared.
I’m a bit more pissed about this year, as the variation is only due 1/3 because of crypto (still hardly documented for them), but the rest was already properly documented in the declaration (RE acquisition + portfolio performance + income), which means they “broke my balls” for a ~7% undocumented variation. Seems to me that they didn’t even bothered to check the whole thing and asked for a bulk explanation, or as I call it : they’re doing the strict minimum service.
Anyways, a simple message with the detail (xxx asset was value Y last year, it’s now Z : that’s your delta) was enough.
I suspect that this is probably an automated flag that’s triggered if wealth difference from Y-1 to Y is above a given threshold compared to income, or something alike.
Just answer with the facts, which I also suspect they’re not even checking before sending the request : portfolio performance resulted in XX CHF increase, the rest is savings. You mention sending IBKR report as proof : didn’t you join that already with your declaration ? If not, that could partially explain why they’re asking 
If you don’t have any shady things to hide, there’s no reason to sweat it : a few sentences detailing everything clearly will do the job perfectly. Just attach the related reports if they were not already in your initial declaration and it will be fine.
I’ve called, have even be talking directly to the tax officer who sent the letter. He said that this kind of request was open ended on purpose, they want an explanation and some documents to support it, but don’t want to tell exactly what. So while the discussion was fine, it didn’t bring much more new information.
Also, we did the math again with my wife, and noticed that the answer is super obvious from the very first page of the tax report : in 2022 our taxable wealth increased of +40k. In 2023 it increased of +100k, ie 60k more than 2022. But our net income increased of 55k and stocks increased of 10k, plus we decreased our spendings. We even feel we could have done better.
Interesting. As you can read from all the answers, you are not the only one getting a question like this. In the examples I know, the question was more targeted.
In my experience, it will be fine if you explain it accordingly. If they are still not satisfied, you can do another round with some additional documents or comments.
Just fyi, I had this several times in the past. I then did a proper table with realized and unrealized gains and income streams (with the help of my tax advisor). I think it was triggered by a big upswing in valuation (in my case private equity and bitcoins). I responded in detail and they never complained.
Of course, they want to hear it from you. Why would they serve it to you on a silver plate or put words in your mouth? 
If suppose that if you were hiding something or have illegal income, the answers and evidence you provide (and is intended) would support their case for fining you or even having you prosecuted.
As in making the difference between
Couldn’t claim you didn’t know what you (your accountants) were doing.