Interesting point. I have wondered the same thing myself, but I did not give it as much thought as you have. If it has an underlying meaning, I think it was something that Peter Jackson and the producers were trying to get across to the viewers, because in the books, the number of arrows was not specific. If I recall correctly, at the beginning of The Two Towers, Tolkien states that Boromir was "pierced by many arrows" or something along that line. But that is just my two cents. Thanks for bringing that forward! :) Hm....now I am going to be thinking about that when I watch the movies. haha
Author's Response: Well I really hasn't noticed it until my boyfriend and I were watching it. I practically flipped out for not noticing it. And I figured it might've been someting PJ came up with.