pakossa

I recently got a Tacx trainer, this being the first time using a power meter. For about 10 years, however, I have been doing monthly "uphill time trials," whereby I put the time into that formula which, supposedly, determines your power. (I think its mass X height, divided by time (seconds) = Newton meters X 1000.) For that, I've been averaging about 300 - 330 watts for around 13 - 14 minutes. Yet, on the Tacx, I've done just an 8 minute effort (for determining CTS training zones), and can only manage 250 watts. I realize neither of these is going to be 100% accurate, but that seems to be an awfully large discrepancy, especially considering how its the shorter effort is so much lower. I know fatigue starts setting in, but on the Tacx, it seems like the resistance is gradually increasing, or some thing. Or, is it possible something isn't set up right? (Yes, I did put my body weight in.)