What "lazy" programming? Sure, they probably should have put a check in the game that went through every car in your garage and re-calculated its PP value when you updated the game, but that process might have taken a while with a large garage, pissed someone off (menu hang?), who then inevitably resets his console, corrupts the game data and is left at square one...
It is a royal pain to have to click on a car and see if the PP is still the same, probably even more so online, but I think it's not really a case of laziness so much as it is convenience and "safety". Really, it's because of attention to the likely usage patterns and optimal data structuring so that the list generation for the garage view is faster, which has meant that this issue exists in the first place. I.e. the data is only updated when the car is changed or viewed.
Hence, you might even say it's due to over-exuberant programming in the name of performance. But maybe they should have added the option to refresh the data, much as with the thumbnails.