Baseball is a game that produces a lot of erratic variations. There are great players who had terrible seasons.
I'm not arguing that clutch or choke is something pure and undiluted by any means.
But maybe, just maybe Schilling and Ortiz had a little bit more of that quality of mental fortitude or whatever it is that made them very comfortable in big situations. Obviously that doesn't mean you're going to come through every time, because it's impossible.