D1.1 (2006) pg.92 B-L2c-S "1/2 to 1" has a root opening that equals zero, with a "as fit up" tolerance of +1/16, -0. So max root opening would be 1/16". The 1/4" max tolerance is referring to the root face dimension (f), which has a tolerance of +/- 1/16" (as fit up), so that is where you are getting the 5/16".
Jason