Different LAP VAM values in Garmin and TP

Hi all

Last weekend I climbed some mountains and monitored the VAM during the ride. After uploading the ride to TrainingPeaks (and Strava), I realized that the values presented in Garmin are higher than the ones presented in TPs. 

ex: Garmin 954 m/h -> TP 868 m/h 

Garmin 850 m/h -> TP 786 m/h

Strava also has slightly different values but closer to TPs.

Is this a Bug, a setting to adjust or only different calculations perform? 

Cheers, 

AN