I don't think it is rigged because there the random number generator makes a random number and that number has to be greater or less than the programmed number. For example, I am certain some people played pokemon sapphire, ruby, or emerald and when you evolve wurmple to get either a cascoon or silcoon, the random number generator generates X number to see is the value less than Y. It is like the card broker, casino and other random things in the game.