Well, I can't find anything definitive in my, "The Art of Building the Pennsylvania Longrifle" book on the reasoning for it, so I guess we'll just call it "builder's preference. I will admit it would bug the heck out of me not having the lines flow with the finished product.
I'll add that I don't consider "The Art of Building the Pennsylvania Longrifle" as the only source of information when it comes to building these ML's, but it's a handy source.