I think the key word in your statement is "start". I am guessing that they have found that most bows will tune well at that measurement. However, that does not mean that all bows tune there. I have found a variety of different nockset positions to yield good tuning results on a variety of bows. If you think about, the way that the cams or cam is timed probably has alot to do with this...as well as tiller measurements etc.

And anyway you know how those Mathews bows are.....