There are a few things going on here:
- The server will allow submissions up to 4 hours after closure.
- Unless, that is, you've been writing long enough to have saved a draft. You might get a notice saying that the question is closed but your form will still work.
The draft system is what's responsible for the weirder outliers (I've seen hours-long delays before) and is much newer than the up-to-45-second window but this one could simply be a client-side delay issue.