I dont know how they rig other raffles, but from personal experience at HoH in Serramonte Mall in the SF Bay Area.
This is what happened to me:
My friend and I entered the same raffle. He gets called. When he showed up that Saturday to pickup his pair, he saw the list of winners with my name on it. I never received a phone call. They usually are very strict on keeping that list of names a secret. I think an employee ****** up and left it out on the cash register counter. My friend called me and I got down to the mall asap to pickup my pair. They had to give me the pair since I was on their winner list.
I am sure they have other ways around the raffle but this is one way they try to save pairs.