This happens a lot on Online Shooting games.
The reason has to do with Ping and Internet packets.
So you may have headshoted a person on your screen, but it didn't register to the server, therefore you got robbed.
Scrub: "Aim pl0x. 100"
Kid: "Wow, fken tanker"
"yo i hope your not a tanker we only got 30 beers left for 3 people!"