iCloud free space wrong?

I have a subscription to a 20 GB iCloud storage plan, but now something strange is happening.

I received an e-mail from Apple that I almost ran out of space, so when I looked on my iPhone it told me the following things:

* iPhone backup: ~ 7 GB
* Photos: ~ 3 GB
* Documents and data: ~ 350 MB
* Mail 1.6 GB

But if I add that together I get somewhere around 12 GB and not the 19,75 GB Apple tells me.

So where did my 7.75 GB go? Apple now thinks I only have 250 MB left so soon my backups will fail