The behaviour of the green + button at the top left of a window is unpredictable. I found out that option-clicking it will maximise the window to the full screen, every time, completely predictable, at last for non-Apple software (Keynote does not play along).

This has probably all to do with Apple’s full screen mode, the two arrows at the top right side of the screen. I use it rarely because this feature does not work very well with multiple monitors.