Players using elytra appear upright on servers

Players gliding with elytra sometimes appear to remain upright to other players on multiplayer servers, giving the impression of true flight as with creative mode or hacked clients. I have observed this effect on two different servers, with two different players. As one of those players was me, I know this isn't just a case of someone lying about having used hacks. This does not appear to happen every time, and may not be reproducible without extensive trials.