Ohhhhh...I see now, duh. They are snapping at 1/2-block heights, or so it seems. They are 1x2x1 multi-blocks and I tested with wood stairs w/railing which are the same size. Only the lockers did this weird partial-block stuff.
I actually have a theory about this that I need to go test...
Edit to add: Theory confirmed. The problem is that the lockers blocks inherit from block "cntLockersShortOpen" which has this property:
If you comment out that property, then the lockers stop being stupid. (Note I did not remove and re-place these; when I reloaded the world, they had been snapped to full-block heights already.)