I personally understand what you're saying with some people doing insane knockback to you, yet taking no knockback, though, and its frustrating. Sometimes it has to due with ping/server connection. I'm not sure what, but I believe it has something to do with the Server and Client side not syncing up correctly, leading to some people essentially "sticking" in place, as they are just simply lagging. Again though, that's what I think happens, and may not be the actual reason it happens, but I know its a lot based on server Ping and connection.