I found the following on the FAQ section of their website:
How can third party authentication companies authenticate so many different autographs?
To properly authenticate a signature you must have numerous exemplars to do side by side comparisons. That is why at GFA we only authenticate a limited number of autographs and have experts specialized in these signatures.
I guess one of the forensic people over at GFA specializes in the signatures of Nana Visitor & Avery Brooks. The below Avery Brooks/Nana Visitor signed photo is presently on a auction site that I will not name.
AveryBrooks.jpg
AveryBrooks--Rocchi.jpg