[Drdrdrq]

True. Also, there is a fourth variable where you can cut corners (even if it's almost never a good idea): quality.

Great teams can produce much more than mediocre ones, but they too have a limit. When deadline is set too close, one of these 4 things has to give, and it is good to know in advance which one that is, so team can set the priorities accordingly.

[Clubber]

I would say that is part of the scope. The scope being a well tested, functional and relatively bug free software that does x, y and z.

[Terr_]

Unfortunately most business people don't make things like "doesn't corrupt the database when an exception is thrown" into a feature bullet.

So while you can argue it's part of the scope, it's not part of the scope that anybody else seems to think about.