The laptop I'm using right now has a 1366x768 screen, a maximized browser window is 1306x768. I bet many other users of of that size are going to have browser windows that have the full 1366 px width, but then some height <768 px. Details depend on the OS, the size and position of the taskbar/dock/whatever, ... It's not much, but it adds a bit or two of data.

Custom sized ones make you easier to identify only if you don't change them.