My available iCloud storage is lower than it's supposed to be

I signed up for a 50gb iCloud plan on my iPhone 6+. The first few times that I tried to back up my phone, the backup would fail and it would say that it "could not be completed." Even though it didn't back up any files, it said that the available iCloud storage was around 11gb out of 50gb. Then, when the backup started working, I clicked "manage storage" and it said that only about 9gb of data is backed up in iCloud yet the available storage is much, much lower than it's supposed to be.