iOS not using -568h@2x.png

I have 3 images:

test.png
test@2x.png
test-568@2x.png

In IBOutlet, a UIImageView is set to display test.png.

On iPhone 3.5in without retina, it's displaying test.png

On iPhone 3.5in with retina, it's displaying test@2x.png

But on iPhone 4in with retina, it's displaying test@2x.png!!!

What's going on?

Thanks!