As you start learning the game and start winning, AI cars in the game also learn the tricks of the game play. As you have noticed, different level of difficulty comes within the same level automatically.
This is not random, it follows you to make the game more interesting for you. There is no separate indication in the game for this difficulty level, it'll deprive you from the fun, if that is known ;)
One trick to beat that is to go for higher power car so you are always ahead till the AI catches you again :)