jaimie lannister actually sat on the throne after he killed the mad king but Ned showed up and ordered him off. Robert then claimed the throne and everyone just followed him. the robert of 10 years ago is very different from the one now. he used to be a very powerful warrior.
That makes some more sense to the scene where he orders more wine from that boy and tells him to bring his cousin in. Saying something to the effect of how much it must suck for him to have to guard the door while he eats and drinks.