What is this shit about a nexus point? If this is the same Kyle Reese that went back to save Sarah, right up to the point where Matt Smith's character did his nano-mind meld trick on John, then how is this NOT how it's always gone down (Using the original Terminator movie as reference)?
It's because the only way for the timeline in the Terminator franchise to make any sense, all the way back to the original, is for it to be a spiral (well, a helix really) rather than a loop.
What we saw in the original movie was completely impossible. There's just no way for it to have started that way; at some point, Kyle Reese had to have been sent back by a John Conner (or more likely, some other figure) that wasn't his son. But once that event occurred, the whole thing with Reese fathering John became a recurring part of events. We just came into the picture somewhere in the middle of the "time spiral," so we never saw the true origins that created it. At the earliest, we were witnessing the third incarnation of the series of events; the first had to be someone else sending Reese back, the second had to involve Resse knocking up Sarah and producing John, and the third had to be the result of John sending Reese back. Chances are there were lots of other jumps in there, too, but three is the bare minimum.
Now what we see in Genisys is that spiral continuing on. Everything that's changed is a result of previous time jumps, and this movie itself even comes out and says that simply by having both Pops and a T-1000 already screwing with things when that never happened in the first (to our perspective; not the actual original which, again, we've never seen) timeline. Each time a jump happens, every single event changes.
Heck, unless Kyle Reese bangs Sarah at
exactly the same time he jumps back, and
exactly the same sperm impregnates the
exact same egg, it's not even the same John Connor we see each time it happens.
I'm willing to bet this is exactly how Skynet explains things in future movies, assuming there are any. It certainly explains the comments he made in this one, about all the different outcomes and the long distances he's traveled to get there.
Some people like defining that style of time travel as the creation of infinite timelines, but in this series it's always been just one timeline... just one that keeps getting screwed up over and over and over again. And at no point is there an actual paradox, as there was a logical way for it all to have started.
That said, there's a reason Matt Smith was cast for the role as Skynet, even if more as a joke than a legitimate reason.