When measuring sound levels distance is the critical factor. If they are measuring sound levels at max rpm while standing 3 feet away from the back of the car 98dB is going to be easy to achieve if you have test pipes. If they are measuring it from 30 ft away at the side of the track then that same reading would have decayed several dB due to dispersion. Sometimes just driving on the opposite side of the track to where the meters are can help.
Also note that some tracks just go through the motions in order to comply with local regulations and don't actually enforce very heavily unless there is a blatant infraction. I was at one track which had a 98 dB limit at 10ft as measured in the pits with a hand monitor, some of us failed and had to "fix" and retest (read we just revved more slowly the next time). Then halfway through the day they cleared the track and brought out a daytona prototype car which was so insanely loud that you could tell where it was on the track just by listening to the sound echoing off the trees.
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