Earlier quoted context omitted.
It IS universal behaviour. The green button maximises the window to the optimal size as dictated by the application. For a browser this could be full screen. For a word processor this could be just enough to see the page.
For me, the algorithm is too opaque and the results too inconsistent across apps to reason about. You could say, "This is consistent behavior, it sends off a request to mechanical turk for the optimal dimensions then resizes the window when the job is done." And you'd be right that the behavior would be universal, but it's not universal in the sense of consistency where the user will be able to guess what is going to…
That's because the algorithm is implemented by the application's developers, not Apple.