It is likely due to the weird directionality of knockback on the server. Instead of moving in the direction your opponent hits you at, you are hit directly away from where your opponent was when they hit you (not confirmed to work this way, but postulated). Both of these may seem the same, but the latter has an amplified difference when it comes to delayed kb. Since the hit registers some time after the buttons were pressed, the direction you get hit is sometimes vastly different than if it were to register when the buttons were pressed. This is especially true for vertical kb. Because you are falling, you are below your opponent and, as a result, take downwards kb even if they hit you on the same level.
