because you achieve more accuracy with offset vanes over straight.
Not true! I have shot straight feather fletch for years with superb accuracy.
Last season I fletched with a 1-degree offset because of some minor flight irregularities, but have tracked down the real problem and will refletch with straight for this season.
Go here and enter
fletch in the search bracket.
http://domino.htcomp.net/ragsdale.nsf