Tim,
I'm not familiar with that. Do you have a reference? Take a look at Table J2.4 Minimum Size of Fillet Welds in the ASD Manual. The minimum size fillets in this table is dependent on the thicker of the two parts joined. It indicates a 5/16" fillet for material thickness of over 3/4". That's where I get the 5/16" fillet on the 1" thick plate. Also, see Table 5.8 in D1.1. In addition to Table 5.8, Note 2 indicates that the weld size need not exceed the thickness of the thinner part joined. That same statement is made in the ASD Manual. I think what they mean here, for example, is not to put a 5/16" fillet weld along the edge of a 1/4" thick plate. On the other hand, should a 1" thick base plate be welded to a column that has a web thickness of 1/4", the minimum size fillet would be 5/16".