That's the mystery of the butt. Sometimes it's 90 minutes per pound, and sometimes it's longer. It depends on the fat/muscle mix and distribution.
Of course, it could have been your Thermapen. Did you have a GREEN one?
Dbl Bbl Smoker, UDS, ECB, Char-Broil gas grill & charcoal grills, SJ - Certified Moink Baller