I am seeing difference between screen size and resolution on OS X version 10.11.6 to be more precise. When I set resolution to 1440x900, I get screen.width=1440 and screen.height=813 using electron.screen.getPrimaryDisplay().workAreaSize API. Why scree.height is less than actual resolution of 900 pixels?
I would assume because the other 87 pixels are taken up by the menu bar and the Dock. Have you tried to move the Dock to the left or right of your display and see if the width value changes to compensate?