Health app is so inaccurate when using just an iphone

Is there a known issue with an iPhone 12 being able to track steps/kcal/kms when all you are using is the iPhone in your pocket (no link to a watch or external app)

I’ve jogged the exact same distance, on the same course, with the phone in the same pocket of my joggers and for the exact same number of minutes …. And yet the results are so wildly variable.

Example: yesterday 336 kcal for 65 mins, the exact same duration and same course the day before was 611, the day before 703, the day before that 420. I’ve changed no settings (weight, etc).

what is going on? It’s so annoying and so unlike anything else Apple related in my life!