To those of you who replied, thanks. Nice to see that there are pilots out there who haven't lost their sense of humour!
Another explanation I've been given is that 100% is the ideal rotational speed but Frank has given us an extra 4% to give the average pilot a bit more time to get the collective down before you end up with...and I love this word...catastrophic rotor stall. Anyway, appreciate your replies.
Happy flying....