I have a Nuvi 660 and 760T, both with North America 2012.10 maps.
The 760T map was downloaded from Garmin, the 660 I downloaded from this site.

Both units show incorrect mileage to addresses I have saved, always lower miles.
Below is chart of the difference in mileage when I compare to Yahoo Maps, Mapquest, and Google Maps.
Although the three are not all the same, they are close, the Garmin is always significantly less.
When using the Nuvi’s to go to the destinations everything works properly, the problem is when
I view a favorite, It shows the saved favorite as being closer than it actually is.

Does anybody have an idea why it is showing lower mileage?

Miles Miles Miles Miles
Nuvi Yahoo Mapquest Google
Favorite 1 132.1 165.89 186.78 166
Favorite 2 135 198 186 192