Because everyone is from different areas and you were probably on the bad end of the region lag from the host. Lag from just one 3rd world player effects the whole match in inconsistant and often weird ways because of conflicting ISPs, distance traveled to and from host, and just plain old trashy internet connections. All of these things and more factors make for things like seeing one bullet fly on your screen but their screen it was 3. If you had taken cover during that it still would've killed you through the wall because the info got to you late because of lag.
No harm taken bro, on the contrary, I would be happy to know if it was a lag shot rather than a cheese shot. I think your point makes sense, cuz I couldn't figure it out when it happened. I mean, we all seem our fair share of red bars, but one that delivers it one go. That's new to me.