After many many years of hosting servers I came to the conclusion that reg is somehow dependent on a few factors.
1 is the performance of the actual server machine.
2 is rate settings
3 the player's slot
4 the server FPS
5 the server tickrate
Where the performance and FPS is most important!
Now here is the most interesting part however:
I myself have experimented a lot playing in the servers myself, in quite an amount of servers the player slot you join is crucial for getting good registration.
This meaning, when I first joined the game, shots were simply missing, once I had rejoined and got a new UserID/player slot from the server all of a sudden all my bullets start hitting.
One time it took 15 rejoins before I got the good reg but it's a huge difference, it often gets me from scores like 1 kill 20 deaths to 20 kills and 1 death just by rejoining.
Now hit registration has always been a grey area, we do not know exactly what influences it, I've seen servers on the best hardware running the worst servers and the other way around. But as far as I know, there are a LOT of things it depends on.
And like dualcore said, only people that know the game pretty good and have "talent" for games really see/notice the difference. People that just spray a bit won't care about the 2mm difference in their aim, players at advanced/professional level however will notice pretty quick.
A lot of players also just have to blame something for their crap shots
