When a top slab is pushed into the head space of a player, it automatically starts sneaking. But when a bottom slab is pushed into the leg space of a player, it does not automatically start sneaking and instead gets put inside the block that got pushed into the leg space.
This is especially noticeable when pushing upwards a full block that the player is standing on while there is a top slab above the new position, because the player falls a full block unnecessarily. In some arrangements this could even lead to falling through a floor.

I built a machine to show all 8 combinations (horizontal/vertical push into top/bottom block while standing on slab/full block) quickly, the video is attached: https://drive.google.com/file/d/1ZwCjm_ggNxkum4F5UiwFI_Ktw9gJN1yB
Note how the vertical pistons for the bottom blocks both push the player up into the new position where it should sneak, but it doesn't, so it falls back down. Sometimes vertically and horizontally pushing pistons are hard to differentiate, but their end result is pretty much the same anyway.