Google based mapping will give you loads of straight line distances even for corners, and doesn't include you wobbling all over the road, and Google maps doesn't include the extra distance in elevation.
Shouldn't really equate for anything over a mile difference though.
I've had nothing but bad news with the GPS chips in older mobiles (not checked my most recent), and it could be possible that is it dropping signal at certain points for short distances and calculating the straight line distance between your newest point (or even ignoring it completely). GPS will also have dodgy jitters concerning altitude too, you should see the graphs between GPS based and Barometric based altimeters...
Best way to calculate the route is to use some OS based mapping with good elevation data that takes into account the inclines/declines, draw your route on and see what that says.
