That makes no sense. The service itself was sold on the idea of Optimized Synchronization. It doesn't make no sense for a service/app to say:
You're low on space, so we'll delete this photos... But we're going to reserve 11GB of space, just in case.
The OP should check their Deleted Album and make sure there isn't a massive ton of deleted (but to permanently deleted) data in there. Also, show your Hidden Photos Album and make sure you don't have a ton of Photos and videos hidden in there.