A 7-pound butt can easily take 13.5 hours - or 16 or 6 - depending on cooktemps.
Cookers - due to set-up, ambient conditions, wood size and placement, and several other factors alone or in combination with others - cause unevenness in cooking, leading to what many call 'stubborn butts'. Thermometers won't tell you if the cooker is cooking unevenly, but you can usually see it if you look. The meat isn't stubborn. The cooker isn't a kitchen oven and can't be expected to cook like one. But there are things one can do to minimize the chance of uneven cooking, or to deal with it when it occurs.