Looking at the list showed that we hadn't suddenly recorded a bunch of stuff. I went browsing on the Internet and discovered that I wasn't the only person with this problem. Apparently, they changed the calculation so that it only reflects the amount of space on their Cloud DVR and doesn't include space available on the in-my-house DVR. Oh.
As Gretchen observes, it's the stuff on the in-my-house DVR that's important when there's a cable or Internet outage in terms of keeping folks pacified. And it's not at all clear how to tell a particular program to record locally or in the Cloud.
They *will*, of course, be happy to sell you a Cloud DVR upgrade.
It is sad that the only thing worse than Comcast in terms of customer service is AT&T, who I fled about 18 months ago.