Can't sync iphone to itunes; says not enough space but it's double counting content on phone and same content in itunes

My iphone is pretty full but there is still some space.

I tried to sync to update it from itunes and it says it needs 9 more gigs of space to sync, which is ridiculous and wrong. I deleted a ton of content in itunes on my computer and it still says it needs 8 gigs.

So I looked at the content that itunes on my computer says is on my phone - it only sees 2 songs, the last two I downloaded and synced. It doesn't see the rest of the 700 as being on my phone.

I think for some reason the itunes in my computer is not recognizing the identical content on the phone as already downloaded and it thinks it still needs to download all the content from my computer to the phone - thus double counting as it sees that the phone is almost full but isn't recognizing 99% of the itunes content is already on the phone.

Not sure how to get out of this do-loop. I'd hate to erase the content on my phone and start over in case it still won't sync. At least now I have the vast majority of the content I want on the phone on it.