Hi
One of the reasons your storage calculations does not match with what you see from the SharePoint center is because of this:
When you buy licenses (add-ons) for a user and assign it, a certain GB is added to ur SharePoint storage by default and your storage will keep increasing in the admin center, but there is a problem when a license expires or is removed from a user account, that amount of storage is almost meant to be removed from the admin center, so the admin center remains the same amount but internally it will be decreased because a license has been deallocated or expired.

So when the internal clock senses the tenant storage is filled it notifies the admin.

To get an accurate report on your storage, you will need to extract all your tenant licenses and add-ons, currently active, and do the math. This will give you accurate data.

please note that the report you get via excel from the admin center is always accurate but the display on the admin center for the tenant storage might not be accurate as it doesn't synchronize always.

To be perfectly sure, you can open a support ticket with Microsoft and they will get you the exact tenant storage and the breakdown you can also ask how you can calculate your tenant storage based on your licenses and add-ons.

I hope this is helpful.

Thanks.