@Cam its actually very simple.
250ms ping means there is a quarter of a second delay between the player and the server.
a 250ms player will peek a corner and see you (since your position is already sent to him before - the server sends all player positions at all times) before the server will inform you of this peeking.
that player now has 250ms + your ping to shoot at you while you dont even know he is there.
even if you instantly start hiding, because of client-based hit registration, all the shots that scored on the 250ms players view will be counted as legitimate hits. this is why you sometimes get hit through walls - you were hidden, but not quite yet in the view of the shooting player.
this might sound like a huge advantage, and indeed is incredibly annoying when a 200ms+ player uses a sniper rifle, fortunately it works both ways. 250ms ping player will not recieve the information that they are being shot at, in time, when the tables are reversed.
this makes for ‘whoever peeks first’ game, which is a bit annoying, and the main reason why high stakes esports happen on LAN events