FTP Detection/Calculation different between Garmin Edge 1040 (Solar) and Fenix 6X Pro (Solar)

Love to know how the FTP is calculated. I did a 1 hour test on a velodrome last week, and the post ride analysis showed an average power over the hour was, according to Strava, 259W.  The activity summary in Garmin Connect claimed 252W.  When I had finished the ride, and the "new FTP detected" window popped up , telling me it was 243W.  This is the value that is then stored against my profile and is used for my zone calculations.

I get that the FTP may be different when doing less than an hour, and it uses whatever algorithm to work out an estimated FTP, but if you actually do a 1 hour test, I would've thought it could just take that actual average power output over the hour as the actual FTP.  Or do I not understand what FTP actually is?

FTP is your maximum power output while the lactate would not accumulate in your blood during an 1 hour ride.

usually, however, it is measured with a 20 minute test. obviously, you would need blood samples every now and then to measure it.

anything else is just rough estimation, but even to do that, you need a ride with a relative consistent, 20 mins effort near/at to your lactate threshold HR

@peterlip8 That's an obvious bug, your FTP can't be lower than what you actually achieve for one hour. It can be higher in case if you didn't go all out. All the 20 min tests are only workarounds for the sake of not getting to exhausted, only the 1 hour power is the real thing.

@Pascal They changed the FTP calculation in version 25.xx (not in a good way if you ask me) so that the FTP estimation now includes not only the last ride but former rides too. The result is that your estimated FTP slowly drops or increases. That means you have to make more all out rides so that the calculated FTP approaches your real FTP over time.