Underground’s AI was programmed so that their vehicles upgrades were based on the player’s, or they try and keep on par with you. However, people found out this also applied to downgrading your vehicle, or if you made your car slower, the AI would do the same. People would take advantage of that by doing that so that more difficult tracks with lots of obstacles to crash into were easier to play, and since the AI cars were never truly faster than you, it worked.