iPad displays incorrect amount of free storage space 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.
I would suggest you do a backup of your iPad and restore, however it may be that what iTunes has access to makes it look that there is more storage than there is.

Apple seems to have issues calculating free space on iOS devices, they always have...

Instructions on Apple.com on how to backup and restore a device