I did the Kiss Europe race and received an FTP notification from zwift of 245 w , at my weight that is 3.95 w/kg , ZP showed my 95%0f 20 min at 3.79 watts , strava and TP 20 min power are 255 watts which equates to 3.90 w/kg , during the race I was close to 260 watts in the first 20 minutes of the race going from my Garmin 520 , Why is there so much difference ? all the power readings come from the same source .