I have always wondered about this too dotty.
I have seen three answers on the same thread all inside 3 minutes? As you say, seems selective / or it is not always active?
Is this 'safeguard' just relating to answers or includes question(s).
The safeguard may be intended to stop one ABer from 'dominating' answers especially in a 'busy' (well it used to be) Topic like Q&P.
Whatever - if there is a rule, then it should be spelt out for the knowledge and awareness of all ABers?