To make matters even more complicated, RWGPS offers you a choice of base maps. And depending on which map you use, it will calculate different routes and will get different elevations.
For example, if re-create
@Alex321 's favourite, Witch's Hill above then if I use Google maps I get an average gradient of 10.4% with a max of 18.3%. But if I use OSM cycle I get a gentle average of 7.6% with a max of 10.4%. Bear in mind that according to Veloviewer it has an average of 12.0% and a max of 27.9% Your OSM Cycle user is in for a bit of a shock!
So the tool is only as good as its input. Clearly google maps has better base elevation data than OSM Cycle.
(btw OSM Cycle is just OSM with cycleways and stuff shown. I get the same elevation results if I use plain OSM.)
Google Maps
OSM Cycle