It’s physics.  When lapping, you have to flip the top and bottom every
cycle, because the top wears concave, and the bottom wears convex.  In
fact, any wear surfaces, like machine ways do the same.  Gravity is pulling
down on the top part as it slides and  overhangs the bottom part, wearing
the edge of the bottom anything, and the center of the top anything (eg a
topslide. )   It’s how you get the concave surfaces you want when making a
telescope mirror, by keeping the lap under and the glass on top.  So
anything that gets pushed over the edges will eventually get a concave
bottom.  Plus, this is a common fault in wood planing, creating a banana
instead of a flat plane.
Cheers
Claudio