This may be obvious to you already, but the source of the additional fuel consumption comes from a few sources (in my interpretation anyway) and calling it "on-road" and "off-road" actually confuses the issue. For the game's purposes "on-road" is only possible in a hex that has a marked road. You can, of course, choose to drive "off-road" in such a hex. In this case you are staying off the fastest and most efficient routes -- and that's where the extra fuel burn comes in. Does this mean there aren't roads everywhere else? No, of course not. I think you would have a pretty hard time finding any 10km area of Poland that doesn't have roads. But these are going to be secondary roads, or worse. Dirt/mud in many cases, rather than pavement. Some of them probably heavily rutted, completely washed out, blocked by abandoned vehicles or roadblocks (manned or abandoned, trees dropped across them, etc). Moving on them is going to be inefficient at best and a real obstacle at worse.
The game by default seems to assume that main highways are not this way. In some places (where they were recently cleared for an advance by a division, say) that might be true. In other places I'd think those highways could actually be much worse than secondary roads -- prime ambush points, mined, struck by any number of munitions, you name it.
And then there's true off-roading (for instance, through the woods or just across the fields and so on). I actually think this would be largely impossible for many vehicles in Poland's forests, but if feasible at all it's obvious why it takes so much more time and fuel. Bogging is a likely risk, moving slowly due to security and the possibility of UXO/mines is a real issue, etc.