The 95-grain 6mm Ballistic Tip was designed by Gail Root, who was the head ballistician at Nosler for a number of years. He wanted a BT that would handle big game, and killed several elk with it.
I don't know if the 90-grain BT is constructed as heavily, but it is listed among the "hunting" instead of "varmint" BT's. Nosler has something of the same marketing theme as Leupold: A product for everybody's taste. So why not a 90-grain?
I would also bet the 90 just might stabilize in a 1-12 twist, while the 95 won't. I have some on hand, and will do some measurements and run it through a couple of bullet stabilization programs and see what comes out.
