Why are we using @2x for iPhone 5 and 6 if their screens are different?

I'm working on an iPhone app supporting iPhone 5, 6 and 6 plus. The design includes some images that almost fit the width of the screen, according to references I have read the images are supposed to be 100% for 1x, 200% for 2x and 300% for 3x.

We are resizing the images based on the 1x for iPhone 6 and 6 plus, but they look a bit smaller than I expected. The sizes are fine, the devices are showing the corresponding ones.

My question is why are iPhone 5 and 6 using the same @2x images if their screens are different sizes? and images will look smaller on iPhone 6 and based on this 6 plus ones will look even smaller...