Fenix 5 shows/calculates completely wrong total ascent

Made a test-ride (mountainbike) withe Fenix 5 and Edge 500:

Fenix 5: Ascent 722 hm (this time I tried automatic calibration on ...with "off" I got already wrong results).
Edge 500: 792 hm (Edge was always good in this point)
Calculated manually from the altimeter-profil (recorded by Fenix): 776 hm

This means: Fenix records the altimeter pretty correctly, but it calculates the acent horrible wrong. This is too much difference. What the heck is Garmin doing??

Any ideas??

Edit: Basecamp shows 798 hm (with the gpx-file recorded by the Fenix 5). Unbelievable ....does the Fenix 5 "smooth" the profile and calculate wrong ascent???