Why do iOS scales my sprites while they are working fine on Unity Editor and Android?

Hello, while developing a 2D sprite animation in iPad I have a strange scaling problem as displayed in the video:

http://www.youtube.com/watch?v=MXLsQXFwj5M&feature=youtu.be

This is a 10 frame animation and somehow iPad scales the sprites. In unity editor or on android I have no such problems, the sprites just render perfectly.

To introduce the problem a bit more, I render the animation from a 3D model in blender, in 256x256 resolution. If I directly use these images as my sprites I have no scaling problem even on iPad. However, for my case the texture size is too much so I use shoebox to trim the transparent parts around the horse. Of course this changes the pivot point, and I do have a small editor script to set the pivot points correctly. When I run the animation with this setting, as I said no problems on unity editor or android, just the problem displayed in the video for iPad. I do import the sprite pack generated by shoebox as a sprite, selected bilinear, 4096x4096 true colour.

One thing I do suspect is my sprite size is not POT or square, if I change the texture size to POT square the horizontal scaling is gone, but the vertical scaling still exits, so it also does not solve my problem. So somehow the problem is specific to iOS and has something to do with auto scaling to POT square.

Any idea what exactly is causing this, and ways to fix the problem?

Thanks,