It's just a play on words that sellers(including moi) have used to their advantage over the years. As far as i'm concerned Short Printed cards should only be applied to items such as those that were issued (or not) like in the 20's and 30's that were deliberately held back or truly short printed to avoid a large issuance of prizes.
Single printed cards are just that, they were not short printed they were just issued in less abundance as compared to double printed cards.
The problem is the counter argument such as ~ well .... then they were short printed in comparison the the double prints .... right?? LOL
To complicate things even further short and single ... both can have the "SP" notation.
The 1957 set PSA pop report seems fairly consistent throughout. There were no short prints ~ what would be the point ??
Who's on first ........