The difference is because it is easier to say this is the type of lumber/timber we want to call #2 and test them for the design values to used. Then to determine what the visual requirements are required to get a set design values for each size/grade of lumber/timber.
So we have a #2 grade that is easy to visually apply, by the grading inspectors, to a group of lumber/timber members with a lot of design values that the engineers get to use. Rather than one set of design values for each grade, for the engineers, with a difference set of grading rules for each grade/size of the lumber/timber members, for the grading inspectors.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.