Not uncommon at all. I always figure on 2 hours per lb when I am estimating time. If it finishes early, drop the smoker down to 140 degrees (food safe temp) double wrap in foil and hold til ready to eat. You can also double wrap in foil and place in a cooler with towels packed on top of it until you are ready to eat. I have held butts this way for as long as 7 hours and the meat was still to hot to pull with your bare hands. Personally I believe the rest is the most important step with a pork butt. Always turns out so moist and tender.