Error syncing to iphone5

While syncing my iPhone5 I'm getting a strange error in iTunes.

The iphone cannot be synced because there is not enough free space to hold all the selected items (931mb required, 1.02GB available).

Well... if I have more space available than required, what's causing this error? Initially when I got this error, it was saying I was 50mb short, so I've deleted a lot of data off the phone to free up space. But it seems that regardless of how much I remove, it thinks it's not got enough free space, but the free space available is ALWAYS more than required.

I'm assuming I'm going to have to run a restore, but I'd rather avoid that if I can. Anyone have any ideas on what might cause this and how to resolve it?

I'm syncing from a MBP running the latest Yosemite (10.10.2)

Thanks!