If the platform in Unity is set to iOS for example, Unity automatically resizes texture sizes to become square, because the PVRTC texture format (default format that Unity is using) can handle square textures only. This square limitation does not apply to uncompressed images, that’s what you see here.
If you want to use PVRTC texture compression (or any format with such limitation), it makes sense to provide the source assets in power-of-two and square sizes always. Other option would be to use a different texture format.
I see… re-reading the inspector I see it mentions sprites regardless of texture type (a bug).
Overall I find that interesting to reformat textures but not sprites, it makes sense since texture coordinates are mapped to 0,1 and it was only being a problem because NGUI itself stores atlas coordinates in absolute pixel size 