What was the elevation you ran at? That is another thing that will greatly affect your times. For example, a 15.50 second run at a 3000" elevation would actually have been a 14.92 @ sea level. Elevation sucks.
http://www.smokemup.com/auto_math/nhra_correction_factors.php
http://www.smokemup.com/auto_math/nhra_correction_factors.php