If this piece of information is so crucial, why not report it clearly?
Agreed. It really should be explained clearly in the OP (of the voting topic, that is), otherwise a lot of people are going to be surprised by the outcome, I think...
I'm afraid that at this point it's complicated to fix things: I didn't know this IRV system and many users probably didn't know it either, I actually had to look it up on Wikipedia to see what it was. And since the rules simply ask you to vote up to 5 pizzas, that is what people will do.