Many athletes and drivers begin to lose some of their skill and performance before they stop competing, i.e. reach their maximum experience. Many often achieve great success after not being at their physical peak, athough it's difficult to say that that is reflected in GT5.
There's nothing particularly unusual about it, but the words that PD chose to describe the trophy were not the best choice. I wonder if translation has anything to do with that or if it was just badly written originally? The only thing the experience bar indicates is number of races(and in PD's infinte wisdom, not even his own races).
It's not as if a hockey player's 1,000th game is likely to be considered "the peak of his career" if he won championships in his 382nd and 523rd. I'm not even going to try to wrap my brain around how the number of games his contemporaries participated in would in any way affect him.
Just say to yourself "it's just PD being PD" and do what you have to do to get your trophy if that sort of thing matters to you. Or just let it happen/finish it off if you're close.