The published maximum RAM is usually just the number of RAM slots multiplied by the largest module that exists for those slots at the time the machine is released. Frequently, larger modules are introduced later on, which the machine will handle even though it goes beyond those published specs.

The MacBook Air is a different beast in this respect because it doesn't have any RAM slots. What you buy it with is what it has, forever, unless you're much more comfortable wielding a soldering iron than most people.