Historic Sales data keeps changing

In January I went through and took a snapshot of the monthly sales figures for our highest performing territories, in order to try and run projections of future sales trajectories in these markets.


As it's end of quarter, I went to repeat the same exercise. I noticed that for january in particular, the sales numbers now being reported for all of these markets had reduced substantially - by as much as 30% ($15k).


So I re-ran the reporting for the original date range (all of 2019) and am seeing that every month has seen the sales numbers revised down.


When drilling down to a specific country within the sales reports I'd have expected some shift in numbers due to the rounding to decimal places, but not to this extent.


Has anyone seen this issue before?