I wouldn't worry about this. Wearables are valuable for their relative tracking.
They help you compare yesterday to today and learn things like how your sleep changes when you drink alcohol before bed.
A device that is consistent is reliable. It reproduces its own reading from the same conditions. The most commonly worn wearables are sufficiently reliable to make relative measurement useful.
There are many engineering, data analysis, and algorithm reasons why two brands generate different numbers but that doesn't really change the value much. It's helping people develop intuitions about what helps or hurts their health.