Because reviewers rarely see portable releases as 'massive' in the same way, so they don't get caught up in the hype train and murky review-buying side of things which results in more 'normal' scores. Take GTA:Chinatown Wars as an example - it scored a mighty impressive 94, but GTA IV, which when you put things into perspective is a much worse game - in gameplay and classic GTA terms - scored much higher. That any game can score 98 (is it?) is ludicrous in my opinion.