As you said, you use it in your car, in the middle of the road, with the best possible satellite reception.

I used it in Fukuoka/Hakata, no tall building, wider lane ways. And the software was constantly trying to compensate (using nearby WiFi) so the blue dot was always roaming.

Also with a car navi, the built-in algorithm projects your where about using your current speed, which is not possible when one's walking (as the speed is too low).