iPhone 5: preview before taking a picture inaccurate?

Just the other day, I was (digitally) zooming in on my iPhone on an object. All I saw was a blurry mess (what you would expect from a digital zoom), but I decided to take the picture anyway. I was surprised to find out that the actual picture was much crisper, much sharper.

I've got an example below.

What you see on the left, is the preview before hitting the "photo"-button. What you see on the right, is what you get.

So I was wondering: is this a bug? Or is this something to save computing power (and thus energy)?