In addition to using a very standard font, screenplays have a lot of other very strict formatting guidelines involving use of things like particular whitespace rhythms.
It's the combination of the whole, including and especially the whitespace rhythms (which are still connected to font metrics, of course), that lead to the 1 page is approximately 1 minute guideline metric, because scene descriptions and actions have different whitespace from dialog.
Interestingly, compared to your expectation that a Tarantino screenplay would be more dense than the Fury Road screenplay, it is actually dialog that has a lot more whitespace overall in the screenplay format (they are subject to smaller inner columns) than scene descriptions/action descriptions (which use the full width of the page). Word count wise, action involves a lot more prose than dialog and many of the best action screenplays have more overall word count than a more dialog-heavy screenplay. (What I've seen of the Fury Road screenplay read like a novel, though as is pointed out elsewhere it is fascinating because it was written storyboard first, which is the exception rather than the rule.)
The screenplay format really is a fascinating thing to study. There are some great books out there on the screenplay format and how it came to be what it is today.