People just aren't good enough at being random to play Rock Paper Scissors*.
Most people play Rock first - in fact, most people play rock 36% of the time (compared to 30% paper. The psychology behind that may be laughable, but the maths is fun). Unless, of course, they watch QI, in which case they know that most people play rock too much, so they play paper. Although if they know that you know that they watch QI, they'll expect you to expect them to play paper, so they'll think you'll play scissors. So they'll play rock...
Anyway, regardless of attempted tactics, in any game like this people will unwittingly form patterns. The human brain is hot wired to see and make patterns everywhere; it' an evolutionary thing, apparently our ancestors didn't play a lot of RPS. This results in a species where individuals use the same opening gambit nearly all the time, or tend to play the symbols in the same order, or they still think that playing the same symbol twice in a row will be unexpected. So if you happen to know that individual, they will become predictable.
Throwing Spock Lizard into the mix doesn't help with that - it just takes a little longer to figure out a partner's patterns. But it does lessen the chance of ties. And, most importantly, it encourages dinosaur impressions.
* The Japanese version is called Chief of the village, Tiger, Mother of chief of the village. Catchy, huh?