I don't know what you are cooking on, but the guages on most cooker are notoriously poor and even if they are accurate, they are usually mounted in the wrong place to be of much use.
In any case, hotter would still be plenty juicy. Quite a few folks around here (me being one of them) cook chicken at over 300.
If I had to guess, I would say you were running hotter than you thought you were.
Edit - I see what you were cooking on by your auto sig. and am even more confident now that you were running hotter than you thought.