Yes - that is why training with power is so effective. Your ability to ride in a headwind (powerwise) will be the same as riding in a tail wind. Speed may change, but power is what it is. Again - rather than look for external reasons for the change (which as WR said, it will always fluctuate), look for physiological changes. Weight gain/loss, eating habits, sleep, training schedule. My power numbers one week fell 80 watts after a 5 day stretch of brutally tough riding (not in my training plan...why group rides should not be your only training). I panicked. However, through rest, eating well, and taking it easier on the bike, my legs no longer feel dead, and my power numbers are up. I raced two crits this weekend, and my legs felt okay and last weeks power test proved that.