Calculated versus 20 min test based FTP

Hi, based on my riding data, including my first road race of the season last sunday (as synced with Garmin Connect) in the last 2-3 weeks my calculated FTP was shown as 340 W , whereas the most recent 20 min test (2 days ago) was 289 W . 340 W - it would be a 5 min power range for me.
What could be the reason of such a big difference?
Many thanks
Regards, Szabolcs

I’ve noticed a significant discrepancy as well. For some reason the equiv power is far higher than my NP on some of my activities (races), basicaly a number I would never reach. I asked one of my friends to check out xert and he came to the same conclusion.

Hi Szabolcs, we had a quick look at your profile - not many activities in there. Could you load more? The system needs to establish a proper Peak Power for you, for example. We did a quick manual fix on it, to get it closer, but it still needs more data to learn about you. Send a note to to follow-up.

Hi Dex, for the system to properly understand your full abilities & performances, you’ll need more data as well. Three this year isn’t enough unfortunately.

Dex, when you signature is derived from robust data, such that PP, HIE and TP are well established, you more likely have the capacity to achieve the predicted power from the software, even if you may have never achieved it previously. We hear this sentiment quite frequently from users. Read the glossary on Power Curve to better understand why best efforts measured as average (mean maximal power) are frequently not representative of your true highest possible power output.