I have a Vivoactive 4 and I set my Maximum Heart Rate to 150 (220-My Age). The zones show properly and everything is fine for a few days. After I manually set it, Zone 5 shows as 135-Maximum. The first few bike rides are fine but then it starts to creep up. After 2 rides it shows as 139 - Max. After a few more it shows as 140-max. and after a few more it shows as 145-max. Why is this occurring?