A stat that only looks at series-WINNING games is so inherently biased as to be completely (IMHO) worthless. You can look at these numbers and intepret them just as easily as being "When Boston's best player plays well in potentially clinching games, they tend to win." Well, DUH!
MUCH more infortmative would be to know how Pierce performs in ALL potentially clinching games - not to knock off all the ones that we didn't happen to win in (and in which Pierce may well not have played well, since we lost).
All this stat says is that there's a strong correlation between the team's success and its best player's success. You might interpret this stat as also indicating that Boston's team success is highly dependant on Pierce's play; when he's off, there haven't tended to be too many other players to pick up the slack (which we know is true; Pierce hasn't had much help other than Walker up to this point in his career). That still says a lot more about the team than the player, though.