GPS depends on a variety of factors to work correctly and may not be 100% accurate in all cases. Along with the usual caveats of GPS accuracy, the adapter's placement in your vehicle is a major factor in determining accurate routes. The adapter may not have as clear of a view of the sky as your phone, and we can't hook up to the GPS antenna in your vehicle.
As GPS takes some time to gain a lock, shorter trips are more likely to have issues with route accuracy.
Other components of the trip, including distance and MPG, are calculated separately from the route. This means that if your route is a bit off, the other information about the trip is still accurate.