Tacx Flow Smart 2240 under-reading power vs. power meter pedals

I have suspected for a while that my Flow Smart 2240 is under-reading power as I have done a few Zwift TT races and whilst I class myself as only an average rider I strongly suspected my power was off as my FTP seemed low and I was consistently being beaten by Tri-club team mates who I normally beat quite convincingly in road TT events.  This weekend I borrowed a pedal based power meter and did a custom workout power test on ZWIFT in ERG mode holding 100, 150, 200, 250, 300W to check the calibration of my turbo whilst the power pedal meter was displaying power in a Garmin head unit and the pedal power meter read ~70W higher at all power levels.  Anyone got any ideas on the cause of this or suggestions for what to do about it?  Thanks