1. I don't understand the method in which your doing this but either way it sound like you aren't teleporting to the ledge, right? Mewtwo landing on the ground immediately after teleport and how he lands is pretty crucial to this.
2. I do realize you can shorten grounded teleports. These are tested with various control schemes and after a few hours of testing from what I can say are perfectly horizontal inputs are what I'm using for identifying these distances. Of course there is always the possibility I'm wrong, but before either of us can prove the other wrong we'd need some type of evidence supporting one side.
3. I just tested this on the Wii U and 3DS, both on standard and Omega Battlefield. The distances are still different. If you want I can give you some points on the field in order to test this, and if possible I will find a way to record it and edit a video showing the variants of distances. It's not huge granted, but enough that can turn a possible huge play into an SD.
On Omega Battlefield for the 3DS version, on the right half of the ring, do you notice the third blue block in the middle right before the pattern starts to bottleneck? Thats a reference point for a teleport with no momentum that if you teleport to the ledge, it'll ledge cancel, provided Mewtwo is centered over that square with no previous momentum. Now if you were to start a dash, with Mewtwo's waist being the center of his model, once his waist meets the blue square before the first reference point and you teleport then, he will ledge cancel, where as before with no momentum this square does not give a possibility to ledge cancel at all. Now the last point, the blue cube after the bottleneck. If you do a teleport with no momentum here, you will suicide to your death attempting to ledge cancel. However, if you run from the nearby ledge then teleport from that square, all of a sudden it no longer kills you and you get a perfect ledge cancel.
I'm not saying the distances are that far, but it is enough to screw with the positioning enough that can mean the difference of an early lead or an early SD. The main reason I was testing pre momentum teleport distances in the first place was because in a real match you likely won't be standing still for one reason or another.