the answer is simple, they are a better athlete and lacrosse player. There are MANY freshman players in college that played one position in high school and then went to college and the coach saw that they can help the team just by being on the field. If this is happening in college, why wouldn't it happen in High school. you want the best players on the field and sometimes an attackman is more talented than most middies and the coach puts him in that role because he can make more of an impact. Syracuse had the leading alltime scorer in high school running defensive middie his freshman year and now he is still running middie. Johns Hopkins top middie was an all American attackman in high school and the coach ran him at middie his freshman year and he is in his 3rd year and runs the top midfield line.
Bottom line- if a player is good, he can contribute more in a different role than someone who has been playing that same position but doesn't have the same skill set.