The GPS in the radio is not as accurate as my cars' GPS

 

The accuracy of any GPS fix is mostly determined by the number of satellites the receiver has visibility of. For GPS, generally the minimum is three. 

Devices like mobile phones and tablet computers use your Wi-Fi signal to orientate themselves when indoors. The theory goes, that if it can see your Wi-Fi SSID, then it must be in your home. It knows where you live because at some point you were outside while connected to Wi-Fi.

In-car navigation systems, use a feature called snap-to-roadway. So even if their GPS fix is bad, the navigation system takes a guess and says the car can only be on the nearest road - which in most cases, it is. This sets the expectation for the average (non-technical) user.

The GPS receiver in any two-way radio does not use Wi-Fi and snap-to-roadway to "simulate" accuracy and so its position will be displayed on the map according to the fix it has - good or bad. So for the average user, the performance will seem bad in marginal conditions.

The GPS receiver in a two-way radio is as good as (or even better than) the receiver a Sat-Nav uses, so when it shows you off the road, that's the best estimate it can give.