Length of gaskin as compared to length from stifle to point of hip, often coupled with a lack of angulation in the stifle joint. Usualy indicitive of a horse unable to correctly get under themselves in the trot, making collection (and thus passage and piaffe), and jumping more difficult than it should be. The "deer jump" like Nick can have is common among horses with this. Clearly, she has compensated for his though, she has no trouble getting under herself and I have no doubt that she could piaffe.
I wouldn't have looked twice at Tin having it (as you say, most babies get that way) except for Nick & Phantom sharing that trait.
Again, its one of those "how much is two much". A little bit exaggerates any over track (in, say Tenn. Walkers), or that big flat halter trot in Arabs. Too much is supposed to get in the way. Except these horses don't seem to listen to what they aren't supposed to be able to do.

I'll see if I can ply with photoshop and add some lines & angles to photos when I bored this week because Alyse is visiting family.