Reading heartrate outside of an activity is inaccurate?

I noticed this quite a few times, when I am on Zwift and wearing my 255 the HR reading doesn't match at all.

I'm on Zwift using a Polar Verity Sense (strap).

The 255 is just on my wrist but there is no activity being recorded. You can see that the data just doesn't match at all, and this is not the only time.



I have recorded a short video here as you can see here.

https://youtube.com/shorts/IoVvgf5ACMw