If you compare the track distance recorded by myTracks with those recorded by other apps you may detect a discrepancy. The general problem is the inaccuracy of GPS. So every GPS track point has some kind of inaccuracy. If you walk a straight line one GPS location may be left of your exact position and the next one on the right. myTracks simply accumulates the distances between these track points. There is no kind of smoothing. Even if you rest at one point the GPS position may move around a bit.
Beginning with version 7.2 myTracks will provide both a smoothing algorithm and also a detection if the iPhone is in rest. In general this will result in shorter tracks. You can active this in the recording settings.