I think the issue is that some of the time (not all), the differences between versions can only be shown in a static image, as when they are in motion it's incredibly hard to tell the difference. CoD is the game where the difference is probably most obvious, but in a lot of other games I think it would take screenshots to be able to notice. As we don't play games frame by frame, I think that's why some people say the difference isn't that big.
Oh yeah they definitely emphasise the difference but there isn't much option. I mean to my eyes, I could spot 720/1080 a mile off 100% of the time but I know some others wouldn't notice but that doesn't detract from the fact that the console is struggling to push performance anywhere near the PS4.
p.s. I ninja edited above as I remembered a debate about a month ago before release on a similar subject.