@kaschmtom first of all, do your watch have baro? If not, the ascent is measured with the GPS and only when it’s set to best accuracy. So, if the Suunto doesn’t have baro and the Garmin have is normal that you see some differences. If the Suunto has baro probably the Garmin is wrong.
Secondly, could you give more data regarding the differences, I mean, real values like 20 m difference or 300 m…