Hey,
when you say you “moved per to qgis”, I assume you were using maps.openrouteservice.org before and are now using the QGIS plugin ORS Tools.
If not, please correct me.
In that case, if you send the same isochrone request (i.e. same starting point and same range), you should get the same isochrone and thus the same population.
If that is not the case, please provide both requests - via screenshots or via the Log Messages panel (QGIS) and the Dev Tools Network tab (maps client).
What do you mean by
Have a look here for a short comment on road speeds used by openrouteservice.
There should be more info on this topic in the forum.
Best regards.