When worked with pie graphs with single (easier to notice) or several items I've noticed that the graph may request 2 times more history that an item has in "keep history" settings.
When look to code (CPieGraphDraw.php:181) in v2.4.6 we can see line:
which is identical to main part of another code line (CLineGraphDraw.php:235), which is for normal/stacked graphs.
It's possible to notice in bottom right corner of graph - Data from history or trends.
The behavior plus almost duplicated code line - it looks do me like a bug.