I've got two iPad Air, one 64GB the other 16GB. Both on iOS 7.1.2.
On my 64GB iPad, the sum of memory used by all Apps very closely matches the memory used figure above the list of Apps (40.3GB vs. 41.2GB). On the 16GB iPad, the sum of memory used by all Apps amounts to ~720MB but memory used is reported as 7GB.
How do I find out what is using the ~6GB? It can't be iOS itself, is it? I guess, I would expect to see a ~6GB difference between the numbers on the 64GB iPad, too, if it was iOS.