All looks perfect. Signal levels are good, antenna is good, PDOP value of <4 is great. I've seen problems when PDOP has been around 12 - 15.

PDOP measurements which are less than 4 gives the best accuracy (under 1 meter). Between 4 and 8 gives acceptable accuracy. Greater than 8 gives poor accuracy.

Remember: accuracy is a combination of
1. GPS calculated location - which can be affected by atmospherics, and is very much afffected by current constellation. It is also affected by surrounding environment (GPS reception in mountain valleys is MUCH harder and is MUCH more inaccurate than GPS reception on a flat plain)
2. Vehicle location (as measured by wheel sensors & gyro)
3. Map accuracy (the digitised map, are the streets REALLY mapped in the right place?)

All 3 of these can be out, the GPS is constantly trying to determine which position is "more" accurate than the others.

For example, atmospherics can cause the following positional inaccuracy:
Ionosphere 0-30 meters
Troposphere 0-30 meters
Measurement Noise 0-10 meters
Ephemeris Data 1-5 meters
Clock Drift 0-1.5 meters
Multipath 0-1 meter

I've seen all of this happen. With experience I know that rainy misty days are bad :-)

As for the map, how do you know the map is right...??

PS: for MAXIMUM nav accuracy, ensure it is using the instantaeneous wheel speed signals, not the averaged speedo signal.