Apple Watch - Heart App - Calibaration

Have a question regarding Apple Watch - Heart App Calibration. Using Heart App continuously for 20 minutes, during the interval of 20 minutes how often does the app calibrate ? Is there a mechanism to measure the calibration.

The reason to ask is the watch had shown impossible result of 39 during rest after walking for just 10 minutes.

Also there were similar incidents of 46 shown for more than 3 days during normal recording of range.

Can someone please clarify on this.