I wrote in in another thread, same thing as happened to OP happened to me; 3 currencies (EUR, USD and one non-major currency that usually takes a day longer to settle), deposit in EUR, wanted to convert those and buy USD asset (t-bills).
I converted the EUR to USD as usual (settles within a few hours usually, in my experience), sized the order so that it would match the USD, and then got the same sequence of trades and conversions that can be seen in OPs screenshots. Similarly, my third currency was converted too instead of sticking with the USD, and I was left with a residual amount of USD in my account.
I guess the priority here is given to settled balances as opposed to unsettled, irrespective of whether that makes sense from the perspective of the trade (which was in USD).
Also, I don’t really understand the sequence of the buying and selling which happens within one second. I noticed the difference in conversion rate and amount, and I think this spread is what IBKR is pocketing for the service. It was 0,0001 in my case for EURUSD (I forgot what it was for the third currency).
I see the advantages of this service (specially if you want to get your trade done as quickly as possible, for whatever reason), but it is frustratingly intransparent. Moreover, there should be an opt-out option for people who don’t need it at all.
The second time, I used this service on purpose, and while the costs are what they are (I think, at least), I got to do the whole deposit → trade-sequece within a very short amount of time, without even bothering with manual conversion. Obviously, there was nothing left of my third currency in the account, so it was more transparent.