What is the inherent problem with pleonasms? In the ATM case (which I originally learned as an initialism for Asynchronous Transfer Mode) nobody would say automated teller machine machine (except jocularly), but something in a person's language instinct drives them to say ATM machine. It could be as Faldo suggests to try and resolve some possible ambiguity, which case works well for PIN. Sometimes a redundancy is just a redundancy.

Ceci n'est pas un seing.