Inexplicable email from Apple warning of limited iCloud storage

I received a legitimate-looking email from Apple--where all of the links actually went to apple.com, not to anything sketchy--stating that only 349 MB were left in my iCloud storage, and that I needed to upgrade. When I checked my iCloud storage, there is almost 10GB left. An Apple advisor verified that I do in fact have lots of room in iCloud storage.

No one at Apple Support had any idea why I had received this email, but they thought it wasn't phishing; instead, they told me it might have been a random error.

Has this happened to anyone else? And why is Apple sending random warning emails about iCloud storage?