I am guessing, but ...
The battery level is an integer and therefore there is only 100 possible values. So at 10minute intervals that would mean that showing 1 percentage change every reading would take just under 17hrs to go from fully charged to empty. So perhaps the assumption is that a battery will in general last more than 17 hrs and so there is no need to read the battery level more often than once every 10 minutes because as an integer it will not change value in less than that time.
In my case, with a Samsung Galaxy Watch and everything turned on, I get around 36 to 40hrs of use ( ie. two full working days, with charging every second night ). So that means it is only changing, on average, around 1 percent per 20 odd minutes or so.
It is just a guess though. Perhaps some watches drain quicker than 17 hrs (?).