We can guess what destination is doing based on @Ponder’s side note regarding entity scaling:
Spinning that thought a little further I would say that destination is likely part of the pathfinding/AI - the area in which/next to an AI has to be to be considered “at” the entity.
That being said, the “height” you wish to modify is a hardcoded constant in the AI (sleep_in_bed_adjacent_action.lua; bed_location.y = bed_location.y + 1 - setting it to 50 results in quite weird things). You won’t come far without overloading the AI action.
(Also, for those kind of questions we would have the modding request thingy… if there was a new thread already)
If you want to get really creepy, what actually happens is that it’s playing the animation while the entity is moving from its current location to the bed in a very straight, linear manner. Without the animation, you would see the settler “sliding” onto the bed. The animation merely “hides” that effect right now.