i don't get it

regarding the controversy surrounding the iranian elections ... i have to say that i just don't get it ... i mean if the ruling council has to give its stamp of approval to any candidate who wishes to run for president ... so why would they allegedly rig the elections ... it doesn't make any sense to me ... i mean doesn't the system entail that they're fine with whoever wins ... so again ... what possible motive could they have to tamper with the votes?