I've noticed that when I was using a fleet to attack with an advanced warp bubble (+3 fleet movement) Every attack cost 4 movement points. If I attacked without having an advanced warp bubble in the fleet it was only one.

This seems like it must be a bug, because when I was trying to take out 12 ships defending a planet I ended up taking the ship with the advanced warp bubble out of the fleet so I could make more attacks in a turn.