While entering a nether portal in elytra mode it teleports the player 8x from where they should be.

While going through a portal and teleported in elytra mode, you are teleported around the same coords as when you entered the nether portal. So if you go in at x=320 z=160 instead of going into the nether at around x=40 z=20 you'd be around y=320 z=160. After leaving the portal it sends you 8x as many blocks as you 1st entered the 1st portal at (x=2560 z=1280) This process can be done as many times as possible by the game. See video for further details.