Wow that is strange. And if you put the two actresses together (from the time they were filming I mean) it is totally believable...and sorta obvious haha, but the difference in the two characters and so the way the two of them acted made it unnoticable which is good in a way and makes the lot of us seem entirely guillable in another haha.
I did know that Sarah was the closest to her characters than any of the other teen characters, only being 20 when the series started. (Except for Eliza Dushku who was 18 in season 3...)