Apple Watch HRs are correct on the watch but incorrect in iPhone app

I have been having trouble with the HR zones display from HR data on Apple watch to iPhone app. Last summer I measured my HR max = 175 Resting HR is 57. I just measured my lactate threshold at 150. Zone 2 should be about 125 - 140 bpm. I walked on the treadmill (40 min) while wearing my polar heart monitor which outputs to the treadmill screen. I wore my Apple watch NOT bluetooth connected to strap. So I was comparing the HR readouts between the strap and the watch. THEY MATCHED. My highest HR during the session was about 155bpm, which I sustained for 10 minutes. The graph on my iphone says my highest bpm was 137, and that I was in zone 1 (below 126 bpm) most of the time. I was actually over 140 bpm for at least 15 min. I'm trying to train myself to feel when I am in zone 2 but this data is just not right. How can the watch be measuring correct HR but not transfer that data to the fitness app? Can this be fixed? How?