Apple Watch HR Readings: Watch-sourced vs Phone-sourced readings

I have an Apple Watch Series 10 Ultra that I use to track my HR. When going thru the readings my phone appears to be the source for a reading (as shown below). This phone-sourced reading appears to occur every 30 minutes, with all other readings sourced from the watch. Is this a case of the iPhone "pulling" a HR reading from the watch rather than the iWatch "pushing" it to the phone? These phone-sourced readings sometimes look strange as compared to the watch-sourced readings before and after - I sometimes have watch-sourced reading in 60's followed a minute later by phone-sourced reading in 30's, so I'm wondering about the accuracy of the phone-sourced readings. Any help greatly appreciated!