I first noticed this with the Honda Civic '97, the very first car I bought and I only used it in a couple of races. I was recently awarded another, and found my original had 3bhp more. It has not been modified. Now I have an Impreza Sedan '10 which I upgraded to exactly 345bhp for the rally special events. I took it out for some practice first, then tried to enter, and was denied because it now has 346bhp. I'm not an expert on mechanics and engines... is the game simulating engine run-in or something?