If I'm not totally mistaken, the Carnosaur counts as a monster and therefore confers no extra save to the rider. // Eltharion
A 1 wound mount gives an armour save to the rider, but a multi wound mount counts as 'monstrous' so gives 'other benefits' as described by the rulebook. In this case, the carnosaur is a lot more powerful than say a cold one and can take shooting wounds randomly from the rider, saving him.