Why do textures take double memory on iOS and Android devices?

My game’s memory usage is non-make-sense high, I look up details about memory by Profiler, EVERY textures take double memory on both iOS and Android devices. Take lightmap textues as example:

All my texture import settings are trying to avoid use more memory, read/write flag disabled, no mipmaps, and all kind of compression modes give the same result.

I found out that it happens on low-end iOS devices and most android devices.
Some devices like iPhone5s do not have this issue, is that related with OpenGLES 2.0 or GPU?