Could it have something to do with your build/strength? Larger/stronger riders may not need to rock the bike as much or use their upper body as much to generate power?
I find it easier to put down power and do intervals on the trainer, and I see some of my best numbers... I tend to have more muscle soreness/fatigue after an outdoor ride.
I do train in a cool 62-64 deg basement with a big box fan, so heat isn't an issue.