InteractiveBrokers - incorrect cost basis reported

I recently closed some positions on IB that were a few years old due to rebalancing.Overall no problem, but the cost basis was slightly off. both in per share (~172) and in proceeds (~518), as you can see in a montage from two activity statements from 2022 and 2024 (all numbers in USD):

Obviously the absolute amount of the difference is tiny, but this difference makes it impossible for my bookkeeping system (beancount) to find the correct lot to be closed. I have to manually adjust either the open or close price.

Now this happens for about every sell transaction, that the cost basis numbers are ever so slightly different. I inquired with IBKR support, but no reply so far.

Anyone else observes the same thing? should the cost basis not stay constant over time/ reports?

The cost basis also includes paid fees / received rebates incurred by the opening trade. It is actually

- (negative transaction value - fees and commissions + rebates) / number of shares

It is not the price at which the trade was executed.

I don’t think IB factors in distributions by a security.


IB just confirmed this. It totally does not make sense to me - how is a bookkeeping system supposed to find the correct open position for closure, if the trade prices are changing??

Is there some other data field that contains this information? Am I looking at the wrong place?^^

I strongly expected exactly matching numbers