Unity imports texture problem

I have a texture size 256x128. It works correctly in window/android platform. But when I switched to iOS platform, Unity automatically scaled this texture to 256x256 (the original file was still 256x128). I checked the non power of two configuration. But it couldn’t be changed (it was grey) because this texture was already power of two. The result was that my texture had wrong ratio. What’s the problem? I’m really confused because this didn’t occur on other platform.

IOS uses PVRTC compression which requires the textures to be square and power-of-two. Android and desktop both use different compression methods, although android can use pvrtc as well. All the details are in the docs :1

I guess it’s a bug that the power-of-two texture setting is greyed out for iOS though, I’m getting it here too.