Even so, at some point you or someone else will want an answer to this. It happens when you make purchases at the ATS store, such as background, or
RATS access. Often you will have a lot of points in one area, but only a few in BTS.
The "cost" of purchasing is deducted from the total points, and so some is taken from BTS, which can leave a negative balance in that one area.
It confused me when I first noticed it, so your reaction is natural.