It’s because you have your Display & Brightness settings Display Zoom View set to “Zoomed.”
If you take screenshots for web development purposes, it’s important to know you’re looking at accurate screen dimensions.
The zoom feature has an unexpected beneficial side effect for web development. An iPhone 6 can double as an iPhone5/5s for previewing screens and taking screenshots. In zoomed mode, 640×1136 (translating to 320×568 pixel dimensions) is the same as the iphone 5/5s. This can also be handy if you have an Axure prototype that is set up for either 320×568 or 375×667.
Here’s one of my favorite sites for iphone screen dimensions: