Remarried (after divorce) = not married, don’t have sex.
Remarried (after widowhood) = married, please have sex.
Remarried (after annulment) = married for the first time.
Concise.
The question is why can some get an annulment and others can’t?
An annulment is a declaration that a marriage was null from the beginning.
When people seek an annulment and it is not granted, the reason is that no invalidating fact was discovered.