Thanks to the folks who responded. Ultimately, the problem was, similar to what @bkc56 suggested, that the link in the email shows the storage used in whatever Google account you happen to be logged into. I was not out of space in my normal day-to-day Google account, rather, in an another account that I don't often log into, which forwards email to my normal account.
If anyone at Google is listening, what contributed to my confusion is that the email doesn't mention anywhere which account is out of storage. I looked at the email address it was addressed to, but the email had been forwarded from the original account, so that threw me off.
Problem solved. Thanks folks.
-Cris