I've never seen a discrepancy like that when using gmap-pedometer. (That's what I use and I've found it "friendlier" than MapMyRun but it may just be me). Usually really close.
Sometimes Google Earth is a lot better than gmap-pedometer, because you can go into historical images and pick one where the (deciduous) trees are leafless, giving a better view of just where that curb is that we're trying to hug.