Yes, it's a bit misleading.
I am in the same boat. 😀
My Outlook account was 14,7GB/15GB full
My OneCloud account was 4.9/5GB full. I have nothing in the cloud, all of it was mail attachments.
I archived 8montts of mail onto my PC.
This made my Outlook account fullness drop to 12.5/15GB (83% full), while the OneDrive only dropped to 14.7/15GB (93% full).
I don't know where MS stores singular data, but the way the verbiage goes (they count attachments towards the OneCloud storage as well as the Outlook storage, that will soon force you into: deleting, quitting, archiving or: hiring more OneDrive storage, which i am sure is the goal here. (Their basic account currently seems to cost 99$/year).
For reference:
I have another e-mail account at Gmail.
The way it works there is : i reached the upper limit of email storage. Deleted 3 years worth of mail in my Outlook app, where i handle all my e+mail accounts. Clearly the number and size of e-maila dropped considerably. However, when i go to my Gmail account online, the number of mail correctly dropped, but tye size of occupied storage doesn't.
For all incoming mail it correctly rises, bit for all deleted mail it never drops. To save my mail i was forced to get some paid storage extension first.
Then i searched online and found similar cases, bit it seems the majority of people didn't notice. I contacted and was working with Gmail support. A few times online for many hours, to help them "test".
To this day we haven't resolved, and i ran out of time for "testing", which is not my job in the first place.
So it seems all these providers have their ways of "convincing you that you need more storage".