Bella is a teenager.  It is not uncommon for a teenager to sit on the floor. It is actually a more comfortable height for a coffee table if there is food on it.

She is the only one in the room who needs to eat food, so there was no reason to move from the floor to another spot like the bed or a chair...which of course would have been more uncomfortable to eat since there is no real table or chairs in the room.

I am not sure why Meyer included this detail except to show us just how out of sorts and on "auto-pilot" Bella is. She seems completely at ease with her vampire chaperones and non-plussed at the fact that she needs food while they do not.