As a couple of other member have mentioned in other posts, the change in elevation within your commute can change your mileage numbers. If your curious on what yours is, here's what you can to do to figure it out.
1. Go to Google Maps
2. Input your start and end point
3. Get your directions
4. Click on the Link Icon to get the GPS data from the route.
5. Go here. GPS Visualizer profile input form
6. Paste the data into the URL field
7. Set your preferences for metric/us
8. Click on draw the profile and see what you come up with.
Post what your results are. I think it would be interesting to see what your daily commute is with elevation changes compared to mileage results.
No I don't live on top of Mt Washington. I thought it would be interesting to see the elevation map from the highest point in the Northeast to Sea Level.