When syncing iOS devices to iTunes, the memory allocations shown in iTunes are all over the place

Question

There must be threads about this, but apologies for my inability to find any.
As the title says: I plug my iPhone or iPad into my iMac, and when I look at the memory allocation as displayed in iTunes, 6GB or whatever are free. Then I sync again, and suddenly 8GB are free. Sync again, and now 2GB are free - even though nothing has changed. I get this with both iOS devices.
Additionally, often what iTunes shows has no relationship to what the device says when I go into Settings: last week iTunes believed there were 0GB in Photos - because after all I had erased them! - but the iPad recorded 25GB, somehow.
Is this a known issue, or is something screwy with my system?
Thanks in advance!

Answer 1

Thanks, but that's not really the issue. I do manage it manually - I've got a lot of smart lists, smart photo albums, and so on, all set up to sync with the iOS devices.
The problem is that the iPhone/iPad memory, as displayed in iTunes when the device is plugged in, seems to be very inaccurate and variable. When I plug the iPhone or iPad in, iTunes tells me (for example) that, with current settings, 5GB will be free. It syncs, and then suddenly 2GB are free. I sync again - changing nothing, so there should be no change in memory allocation - and suddenly 6GB are free. Somehow, despite syncing no books at all, there's a bizarre 60MB "Book" that iTunes tells me is sometimes on the iPhone, and sometimes not. That little stretch of purple just suddenly appears some days.
At times it tells me it's too full and can't sync at all. So I wipe, for example, 6GB off the "too full" iOS device, and after doing that suddenly 10GB are free, so I put the 6GB back on no problem. And after I add that 6GB back on, maybe there are 8GB free, or maybe 2GB, or maybe 4GB. In my world, 10GB-6GB=4GB, but iTunes is using quantum arithmetic or something.
Wiping a lot of photos off was great fun. I had iTunes unsync and remove all photos - so, in that line at the bottom, all the photo memory allocation disappeared. Cool. Mission accomplished. Then I clicked to another tab in iTunes, and suddenly the photo memory jumped back up to 15GB. Looked back in the Photos tab - nope, I'd told it to sync no photos at all. Checked the iPhone itself: oh, neat. 25GB of photos, despite not syncing any. Eventually, after repeating the process several times, all the photos were eventually gone, but the whole process was bizarrely complicated.
Basically, when I plug the iPhone or iPad into iTunes, iTunes seems to have no idea what the memory allocation actually is - which occasionally makes syncing complicated.
It's not a giant issue, but it's quite annoying - and I'm wondering if this is just me, or if this is a common issue. I think this issue may also be discussed in some of the "can't delete songs" or "can't delete photos" threads, but some of those threads are so long and cover so many different problems that they're kind of hard to read.