Difference kilometers GeoField and Google Maps

Hello,

i have two field Geo-field-googlemap

I calculate kilometers between two city, width View

I use my first field - proximity, and in Source of Origin Point (Other GeoField) and i select my second field.

Unit of Measure : Kilometers

There is indeed a calculation is done, the greater the distance, the greater the number of kilometers is big, but it still lacks several kilometers compared to the real Google Maps Distance

Distance between Quebec and Montreal

With my view : 229 km
With google maps : 250 km

Distance between Quebec and Vancouver

With my view : 3,790 km
With google maps : 5 169 km

What is the problem....i need help, i juste want calculate distance with two field...