Folks who cut their culinary meat-roasting teeth in an oven are often under the mistaken belief than when you set an oven at 250F, it stays there for the entire cook. Nope. Put an oven thermometer on a shelf you can see through the oven window, set 250F on the oven, and watch the temperature vary as much as + or - 20F. My natural gas kitchen oven is + or -10F (a 20F swing) at lower settings, 20F at higher temps (a 40F swing). AND, my bottom shelf is 15F lower than desired setting; the top shelf minus 5F lower.
+ or - 25F at a 250 setting in a smoker is no big deal. The meat doesn't care, but the clock does.