iPhone storage almost full, but iTunes says 6GB free...

I have seen a lot of posts on this forum about the "iphone storage almost full" message, but they seem to be different from what I am experiencing, hence another post.

My 16GB iPhone 6 complains that storage is almost full. According to the usage settings, only 1.2GB left.

But when I connect to iTunes, I get the following picture:

According to iTunes, there is still 6.45GB capacity left.

But more interestingly, when I add up the different components (audio, apps etc) plus the free space, I come to 12GB. iTunes reports the iPhone's capacity to be around 12GB, even though it recognizes it as a 16GB iPhone. In short, it seems as if I'm missing out on more then 10GB of the 16GB available on this phone!

If anyone has the same problem, have you been able to deal with this?

I must add that I have already done a full erase and restore.

Thanks