I've noticed the available storage discrepancy myself, even before this latest iOS update.
I believe that his discrepancy between what the iPad reports as available storage and what iTunes states is often discrepant, and has been a constant complaint for years (just do some googling) - NOW, why this is so seems to be unclear (and I'd love to know -

)?
As an example, I updated my 64 GB iPad Air 2 a few days ago - the reported total & free storage is quoted below - both the iPad & my MBPro w/ iTunes shows an iPad w/ 55.9 GB of total storage, BUT a 5+ GB discrepancy in my 'free' storage - don't know why. This iPad was just updated to iOS 8.3 (and I routinely 'Reset' after an update/upgrade).
Now, I have several apps on the iDevice that includes
DeviceStats which reports a value of 26.3 GB free, i.e. in-between the quoted amounts below - scratching my head! Dave
iPad - 55.9 GB total w/ 24.3 GB free
iTunes - 55.9 GB total w/ 29.9 GB free
P.S. For those who may be wondering why my 64 GB iPad is reporting only 55.9 GB - the answer is that a 'Byte is not a Byte' and depends on whether the calculation is based on a base 10 vs. base 2 calculation - using base 10, 1 GB = 1,000,000,000 bytes; however w/ base 2, 1 GB = 1,073,741,824 bytes - thus, dividing 64 by 1.074 = 59.6; yes, another discrepancy which likely relates to the space occupied by the iOS - this indeed can be confusing -
P.S.S. as to my first P.S., more information
HERE.