When you play Fullscreen it ignores everything running in the background and focuses only on the game (that's when when you alt tab you might notice a 1-2 second delay before you get back to the game.
When you play Windowed Borderless this doesn't happen. Everything is taken into account, your background, toolbars, any ads popping up on skype or whatever. (you'll also notice there is no delay when alt tabing.)
What's the reason you want to run Windowed Borderless anyway?
