Currently there are 2 bugs which completely break pvp, and this is one of them.

The knockback distance is determined by the order of players logging in to the server. So, the players that logged in last get the correct knockback, but players who logged in earlier get incorrect knockback.

Steps to reproduce:
1- Setup a command to teleport your TestPlayer to a fixed location, and don't move your test player;
2- Setup some signs to signal the distance that you knock the player back;
3- Relog.
4- Now sprint and hit the player once. Notice how far you send the player. Try this a few times, you should get similar results.
5- Relog your test player. Now, with your main account, repeat step 4. Notice that now the test player is getting the correct knockback (and not being sent "flying"). Again, try a couple times.

According to TrazLander ( https://www.youtube.com/watch?v=kp4h0O_YKYw ), something that might be causing this is something that we think that can be named "Packet Priority", where the packets are not being sent to all players in the correct order. This is a logical explanation as if the player logs in last gets the correct knockback, means that the client is getting the packets in the correct order and faster than other clients.

Yes, this is in vanilla Minecraft. No mods, neither on the server nor on the client.