The main reason for the distance in the time here is that for openrouteservice and GraphHopper you are selecting the HGV profile, whereas for Google and Bing you are using the car profile. In normal circumstances (especially on highways) HGVs travel slower than cars. Changing to a car profile with the above route for openrouteservice will give you a duration of 8 h 31 min - link
In terms of validation, it is often a combination of looking at the routes generated and times and comparing them to ones from Google to make sure that in general they are similar, but of course there are circumstances where there are some large differences. We had an example in America where speeds were’nt placed on ways in OSM data, so the code had to assume a generic global speed for the road as doing spatial calculations to determine the actual country a road is in can add a lot of time to the build time of graphs. Perhaps @andrzej can give more information about testing as he has done quite a bit for various testing for features that we implement.
If you want to know which is more accurate, that is a very difficult thing to determine on a global scale. In Germany (and much of Europe) OSM data is very good so the road network coverage in openrouteservice has a pretty high accuracy. We also have a number of things that attempt to make routes and durations more realistic, but though these work well in most circumstances, there are always some where they have a more detrimental effect. Obviously, companies like Google and Bing have billions of dollars to throw at validation and tweaking algorithms, whereas openrouteservice is a small group of 4 or 5 developers. So for highly validated routing, the larger companies (Bing and Google) are normally your safest bet, whereas we try to provide more cutting edge opensource algorithms and features (e.g. avoid border crossings, prefer green areas for pedestrian) which are free to use and deploy.