If it's always 40 watts it sounds like the intercept is wrong. Have you tried stopping pedalling for 2secs so it re-calibrates? How about calibrating from your garmin? Could also be the powertap that's off. I have both and ran them together once with a 5-10w difference during a TT.
i have both as well and ran them together on different intensity-levels: very close together, difference most of the time between 4-8 w. at around 200w it was a little bit more, ca 6-10w.
in the worst case, you have to send one of your systems to the manufactorer who has to recalibrate it.
there was a case similar like yours, concerning p2max, in german "tour-forum. the users powermeter showed to low numbers. he got it recalibrated by "saxonar", and it worked fine.