[dairylee]

Neither of those cases need to be true for a second PR to be ready before the first has been merged.

For example you do your first PR, mark it ready for review. While doing it you notice there's some refactoring you could do to some tangentially related code. It's very conceivable that the second refactoring PR could be ready pretty quickly.

[delecti]

There's a reason I said "two of the likliest outcomes", and not "the only two possibilities".

Yes there are other situations, but in my experience, the ones I mentioned are the most common.