My iPad Air 1 (8.4) displays wrong amounts of free space.
It displays 2-3 Gb less than my Mac does when connecting the iPad to it.
If my iPad states it has 0 bytes left. My MacBook then sometimes can install Apps on it, but sometimes it also can't.
Also the amount of space the iPad displays varies from time to time, without changing the individual amount assigned to each App. This is in a zone of 500 mb.
3 Gb is called others on my Mac.