Well, if you watch any one of those % numbers as the watch discharges, you will see they
steadily go up as the watch battery discharges (they never go down) So it seems clear they are intended to represent app usage as a fraction of total capacity, not app usage as a fraction of total usage. So we don't ever expect them to add to 100.
Now let's say you are at 90%, then in a perfect world, we expect the numbers to add to 10%, but as you point out they add up less. There can be some errors at work:
1 - every app less than 1% is not even listed. So if you have 10 apps using half percent each and not reported, that's a total of 5% unaccounted for It's more of a noticeable error when the battery is near full then when near discharged.
2 - The % used by each app is only an estimate based on how much each app uses the resources (cpu, internet, gps, etc). These estimates are not exact. In contrast measurement of "actual" battery level at a point in time based on voltage etc is much more exact