Lachy,
I would bet you London to a Brick on, that the reasons are that we generally rebuild Old Sails for our purposes and lower cost than purpose built.

I asked my Sail Maker roughly "Emachisit" his reply was $800 to a Grand. Go figure!! So whatever the angle was to start with will only be altered slightly when remolded.
I have 3 sails and one spare all have different Batten Angles.
Ron