Post/jello cards grading

I have been looking at Post/jello PSA graded lately. I don't however understand how PSA grades the cards. I find alot of cards are graded authentic and others have normal grades. Why is this happening? What is the criteria for authentic/or graded? Which is worth more? My thought is the graded cards. Any help appreciated.
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If I am wrong I apologize but this makes sense at least to me
They all had complete black borders, no creases,clean back, no stains, bright colors. They came back 5,6,7 and one 8. The 1961 Aaron and Spahn (company issued )came back authentic.
I sold them on Ebay. Not sure I'll send any more to be graded.
Also have a few 1962 Jello that still say cut along the line, in great shape. I'm thinking of doing some trimming and than send them off to the dart board for grading.
"Molon Labe"