More likely the credits system runs on eventual consistency, and he hit a different backend.

Don't think so, I expect that system to use Spanner, so my best guess is that the user generated an image at the end of the credit reset window (which is around noon EST).

If there's something I'd expect Google to use a strong consistency model for, it'd be a credit system like that.

Well, not that they don't do stupid things all the time, but having credits live on a system with a weak consistency model would be silly.