As visible on the picture other players sometimes float in the air when they sit.

This caused by 2features glitching with each other:

1, When you sit on something, the game puts your character's ass on the surface.

2, If you try to sit on a surface, but you are too far from it, the game moves your character close to that object.

If the 2 feature doesn't comes into play, ie: you sit when you are right on top of the given object then the above bug doesn't happen, your character correctly displayed for others. It's only bugging, if you try to sit while there is some distance between you and the object you're trying to sit on.