Elevation gain/loss in courses largely overestimated


I have recelty bought a FR 945 and I am new to the Garmin ecosystem. 

I have created a number of road cycling courses both by designing them directly on Garmin Connect and by importing them from Strava via GPX files. In both cases, Garmin Connect overestimates the elevation gain and loss by roughly 20-30%, making e.g. the ClimbPro function totally useless. When creating hiking courses things seem to work fine. 

Any suggestion on how to fix this behaviour?

Thanks a lot!