I'm not a stocks guy and I've never really understood all of it. But I think I heard that they manipulate the S&P 500 by constantly changing which companies it represents. As in the poor performers are dropped out in favor of better performing companies.
So I guess I always thought it was S&Ps "500 companies list", but it's really a "best 500 companies list", or something like that. Which makes perfect sense since you said it covers around 80% of the combined value.