Party Packin'
I had a Noddy party once. While not as awesome, these Hi-5 party packs, avaliable in foodmarts, still make for one fully attended birthday party:

Or... do they? They give you six settings... but how many invites?

So, basically it is assumed that at least two people the hapless kid invites to their party will not come. 3/4 is the MAXIMUM amount of friend-party turnover allowed. No exceptions.
It must be science or something.