Macbook resolution set at 2560 x 1600 somehow?

This might be a really stupid question (apologies if so), but here I go. I've just got a 2015 Macbook, which has a 2304x1440 Retina display. However, when I take a full-screen screenshot, the image is 2560x1600. Also, when I open a 2304x1440 image in Photoshop and display it at 100%, there is a border around the image. Same happens if I set a 2304x1440 image as my wallpaper (using 'centre' setting). If I open a 2560x1600 image at 100%, it perfectly fills the screen, same happens when I set a 2560x1600 image as my wallpaper.

Therefore, it's as if my Mac thinks that it's a 2560x1600 display or something, but if I go to System Report -> Graphics/Displays it shows the resolution as 2304x1440. Apologies if this is completely normal and I'm just not understanding something completely simple, I'm just worried that images or text might not be as sharp as they should be, due to the graphics card thinking the native resolution is something else. Any help would be greatly appreciated!