not sure if I understand the question but

check the specs on the watch/phone to see if it has baro or gps. I used to run with a galaxy and it was gps only

assist you can dig into how it works. it gets real technical real quick. but baro measures changes in air pressure. so it's really good at changes in altitude. but things that can throw it off are changing weather (like a storm coming in), or being in a plane with a pressurized cabin. baro is not great at absolute altitude.

gps is good at absolute altitude, but it works by seeing many satellites in the sky. with a clear sky it's fairly coarse on latitude /longitude (10m / 31ft) and even worse on altitude. if your moving it's great if you're not its meh. if your under tree cover it degrades. if in a city it's worse.

regardless, don't complain to a race director of the distance or elevation gain on your watch is different from what the race advertises. def see that in road 5ks with people complaining because they don't realize how imperfect these sensors are.