That is just the difference in barrel profile. The one you posted, I believe, is an M4 profile (made according to standards used by the military so things like grenade launchers will attach properly) whereas what was posted above is is pretty straight forward. Im not too knowledgable on that basis, but I can tell you that the difference in barrel profile is not too important, at least in my findings.
Perhaps the barrel with a straight profile is designed for a state with a tight AWB in which it cannot have a flash suppressor or magazine that holds more than 30 rounds (MP15 FT comes with 10 round mag).
Regardless, the MP15T has treated me very well. The Troy rail is very well made and takes about 3 minutes to remove from the barrel nut. The iron sights are nice though my front sight's screw likes to back out without lock-tite. I might ask Troy for a replacement screw because mine is starting to wear with the extra tightening, but all in all, I like the aprts very much.