It uses lap 1 to guess how long you'll take to drive any given sector of the track (well more specifically it uses your previous laps, but on lap 2 it's only using lap 1), if you screw around on lap 1 you'll screw up the guesses. For example, it thinks that you stop for 40 seconds on the front straight so it guesses that you'll lose 40 seconds there on lap 2 also. In more equal conditions (both driving as fast as possible) this is the most accurate way to get reasonable gap estimates during actual mistakes (if a car slows down, your gap can immediately narrow, whereas if it just used 'relative times you hit the current sector' it'll take a couple seconds to react). The only better option is what iRacing does: allow no modding, preprogram sector times for every car and every track.