Wood: the next time it happens, ask the guy that shot you if they were looking shot through a wall or hill. Seriously, ask them what they saw. If you ask enough people an get the same sort of answer, you can trust the responses.
I used to think people were doing something "questionable"to get these kind of shots but so far I have found that it depends really on what they are seeing on their end. Remember, damage is applied based on what the FE of the attacker (firing player) sees. If they saw you clearly and saw their shot impact, it does not matter if you could see them clearly or not. It might suck, but it works that way for everyone.