You can find a discussion of the grading system here:

http://foragetesting.org/files/2015SignupForms.pdf

http://foragetesting.org/files/2014SignupForms.pdf

A is the best grade, down through F for failure.

They test for some of the most common run tests and yes, the goal is to have results be comparable.
There are no guarantees that all test results will be the same across labs but the NFTA is the most vigorous certification program out there so your best indication that you have a lab committed to quality control.

Eleanor in PA
www.drkellon.com
EC Co-owner
Feb 2001