Although i don't know for sure, i do believe this is intensional. My dad use to work at a company that imports clocks. When displaying an analogue clock, as an industry standard, they always adjust the hands to 10:10:30. This was believed to be the best way to display an analogue watch or clock. When i was a kid, everytime i visit dad at work and hangout in the display room, ALL the clocks were set to 10:10:30. If you check out all the analogue watch faces on your apple watch, you will notice ALL of them are displayed that way. So i dont think this is an error of glitch.
As for why it’s 10:09:31 and not 10:10:30, i think it is because for a digital time display, apple wants to show more variety of numbers rather than using too many zeros.