I can't say it's not normal, these things have a mind of their own and 2 identical butts can cook very differently in the same conditions. You should definitely check your therms accuracy and/or invest in a remote digital therm. Don't know if you saw this thread started today or not but he has an electric smoker and got the exact opposite results as you at the same (supposed) temps.
http://www.smokingmeatforums.com/t/120694/1st-attempt-at-smoking-led-to-butt-troubles
http://www.smokingmeatforums.com/t/120694/1st-attempt-at-smoking-led-to-butt-troubles