Probably the only thing that saved that player was they were in a ghost ultra, had that been the regular ghost you would have definitely killed them. The ghost Ultras are well armored.
I had this happen with the Railgun the other day. I got caught off guard and should have traded kills with my opponent as I nailed him just as he placed the final headshot on me with his BR. For whatever reason he survived a direct hit from my Railgun.
Wow, that is pretty crazy. But as Lethal mentioned above, it was the ultra that saved him. Don’t get me wrong, I still think he should have been toast there.