iCloud storage doesn't match what is showing on my iPhone storage

I have seen this question before but I have never seen it answered properly....

My iCloud storage is showing as full - and shows I have 23GB of "Documents". However my iCloud drive is empty.

I only have an iPhone connected to the account and the storage settings on the phone do not show any 'documents' - just the amount of storage used by applications, photos, messages, system data etc. So that's no help.

I don't understand why iCloud Storage can't show me what this data is and where it is - so I can work out if I need it or how to delete it. Or why iCloud Storage doesn't match what is showing on my iPhone storage.

Please help !!!

.....And please don't just point me to the link "Check your iCloud storage on any device" because that just says that 'Documents' can be classed as data from a whole host of apps like Notes, Reminders and other 3rd party apps. Which is obviously no help !!

[Re-Titled by Moderator]

Original Title: Documents in iCloud Storage - What is it?