When I shopped with Mama, I remember that cuts were always made in this "baker's dozen" manner. When I studied "home ec" in high school and college, the true grain issue was taught and that accounted for this fudge factor in cutting. Today I rarely find much of a grain discrepancy but, in the distant past, I've run into some doozies. There may be some sellers/cutters who are still passing along the "little bit extra" without realizing the reason this used to be done with yardage sales is no longer a factor. And this might explain our little sense of entitlement to the extra!