I work for a major cell provider and here is my take on this.
Throughout the 6 and 6s series of iPhones and putting on Zagg Tempered glass, here is the only conclusion that I can come up with.
If I am not mistaken, there are a few different companies that make the glass screens for iPhones, and one of the companies, I am not sure who, but the glass they make for the iPhone is more curved than the other company which Apple contracts with.
So the Zagg protector will fit perfectly without halos or rising on one of them, and not the other. I've put on over 300 screen protectors on customer's phones, and some go on great, and some Halo. So the only conclusion I can come up with is what I said above.
It's luck of the draw. You don't know which screen you have on your iPhone until you try to put on a Zagg.