Why results from the same watch with gpx extensions. whether fit. They differ from each other ?

Coros Veritx 2

Ask in GPS Speed Surfing forum

Late reply but I haven't been on here for months.

GPS / GNSS chipsets calculate position, velocity and time. There are numerous methods of calculating speed from the GPS / GNSS signals, but doppler-derived speeds are by far the most common. In this community the speed calculated by the GPS / GNSS chip is therefore referred to as the "Doppler speed". The Doppler-derived speed is far less prone to errors than the positional data.

GPX files from COROS (plus others like Garmin, Suunto, etc) only contain the position and time data. Software reading the GPX files is therefore forced to calculate speed from the error-prone positional data, leading to misleading speeds. However, FIT files include all 3 data components (position, velocity and time) and therefore contain the most accurate estimate of speed.

TLDR - use FIT exports (not GPX) from COROS, Garmin, Suunto, etc.