Wattbike Vs Rally pedal power readings

Hi. I have recently bred using a Wattbike on Zwift and using this to calibrate my FTP etc and set NPI for races, however I can't hit the same numbers on the road using my Rally Pedals. I did a test today but putting the Pedals on the Wattbike, after 90 mins cycling with a couple of mates, the average power reading on the pedals was almost 50 watts lower than the Wattbike?