Also, the military is again messing around with "selective availabilty" for security reasons.
I find mine to be a little unreliable while in the counties surrounding the Ft Stewart military base in Hinesville, GA.
This is based on past use of a Garmin Street Pilot III, and not the one in the Murano, but may be similar.
Also, Cloud cover and moisture content can sometimes weaken the reception of the signals and increase the margin of error.
In the Garmin, you can go to a "GPS MENU" to get a view of which sats are visable, receive signal strength of each, error accuracy in feet, and elevation.
I have seen mine plot a parallel tract to the road I am actually travelling on for a short time after making a turn.
Also, like mentioned previously in this thread, constant infrastructure repair and expansion require periodical software updates; and they usually have a cost associated with them to pay for the map makers that produce the data.