Gap at the bottom of maximized windows
Those 5 pixels are tied to the Dock. You can position the dock on either the left of right side instead, but the gap stays on the side of the Dock. I think this is something fairly low level with the Dock, as window managers (like Spectacle) will maximize windows and not use the gap.

My $0.02: If you have a lot of screen real estate, as it looks like you do, I say shrink some of your windows to a more appropriate size and/or fullscreen windows instead of maximizing them.