Not enough space to sync???

My devices sometimes refuse to sync. I get the message that I don't have enough space on the device and that I need "x" amount of additional space.

But when I look at the space available, it is significantly more than what it says I need. For example, it may tell me I need 12 more GB of space, but I have 15 GB available.

What am I missing?

Thanks for any help.