If you compare the track length recorded by myTracks or the ascending and descending elevation values to those recorded by other apps, you may notice a discrepancy. The general problem is the inaccuracy of GPS. So each GPS track point has some inaccuracy in latitude, longitude and altitude. When walking in a straight line, one GPS location may be to the left of your exact position and the next to the right. myTracks simply sums up the distances between these track points. There is no kind of smoothing. Even if you stay at one point, the GPS position may change slightly.
As of version 7.2, myTracks offers both a smoothing algorithm and a detection of whether the iPhone is at rest. In general, this results in shorter tracks. You can enable this in the recording settings.
The calculation of the ascent and descent values is slightly different as GPS elevation values are even less accurate. The difference between two altitude values must be greater than 10 meters for myTracks to add the difference to either the ascending or descending altitude value. If you ride a route where you always go up and down less than 10 meters, the ascent and descent altitude values will remain at zero. Other apps use different algorithms to calculate the ascending and descending altitude values. Therefore they show different values for the same track.