Email says "Your iCloud storage is almost full." but Settings says otherwise

Every couple of weeks I get email from "iCloud <noreply@email.apple.com>" like this most recent one which says:

Your iCloud storage is almost full. You have 368.9 MB remaining of 5 GB total storage.

Upgrade to 50 GB for $0.99 per month

However when I check iCloud Settings, either on my MBPro (10.13.3), on my iPhone6 (11.2.5), or directly at iCloud.com, they all say I have used only 3.63GB and have 1.36GB available. They have said something close to this consistently for months.

What is going on? I obviously do not want iCloud (e.g., backups) to fail on me, but I would like to think I can trust the Settings information about my usage.

Note:

1. Im pretty sure this email is not a phishing scam, since the salutation uses my full name and all the links are to https://icloud.com/...

2. There have been previous (closed) questions about these "storage almost full emails", but none have specified that direct checking of Settings show consistent numbers quite different from the email message. Replies have focused on explaining what might be using space, how to delete backups, or just suggesting paying for more storage. None of those address this direct stark contrast.