I'm a little late on this discussion, but I must agree with you. I'm in the same bucket with the same stats actually (9-medium, 9.5 when I'm about to stick my girl, and 8.5 when I just jerk off for the hell of it-without any visual stimulus)
My size varies a lot (as much as an inch) as well depending on lots of things ranging from level of arousal, temperature and also how tired or awake I am. To my knowledge this is 100% normal. Don't worry about it!