PSA- Minimum Grade Question

[Filthy]

I made my first ever PSA submission last week, consisting of 6 individual 1956 Topps cards. I had eyeballed them all very well, and estimated them to all be between 3 to 5 PSA. I put a minimum grade of 2 on all cards, as I was confident none of them would be that low, and i wouldn't really care to have anything below that threshhold encapsulated anyways. I submitted, the following 56' Topps cards.

Ted Williams (2)
Hank Aaron
Mickey Mantle
Roberto Clemente
Jackie Robinson

Of the group, the Clemente, and the Mantle were in the best shape..and was hoping for a 5 or 6 on those...and would be happy with anything over a 2 on all the others.. However, after logging in and seeing the given grades, I was somewhat confused. All cards received a 4, with the exception of the Clemente and the Mantle. The Clemente received a 6, and I was stoked about that....however, the Mantle was marked "Minimum Grade?"

I'm not one to question, the grading practices..as they are the experts..but, I am confused as to what could make a card that appears every bit as good as the card that received a 6. There are no scratches, or creases of any kind, and it is well centered. Just curious as to what I could be missing?

2nd/3rd part of the question is...is there any possible way, they missinterpreted the "minimim grade" number, that I gave them? (2) I am somewhat bummed about this deal, but didn't know if it was open for discussion with PSA...or this is just part of the risk? Status Quo?