Remember when people were jumping through hoops trying to justify how the autographs were getting cut off? 
One global truth in collecting is Occam’s razor and the principle of parsimony. When faced with multiple explanations for a phenomenon, the simplest explanation is often the correct one.
Regarding CGC’s Signature Series, I think we can assume that this was added to inflate the value of the cards rather than as confirmation that CGC witnessed every single signature in that series.