Interesting card.
Trimming to fit a page or holder is the most likely explanation, and with no other flaws it's not scrap as we usually think of it.
However - The lower edge is also not straight, and the unevenness somewhat matches the top edge. I'd say only somewhat because of wear, and it may have been a close match when new.
This sort of edge can actually be a factory cut. The blade cuts through the stack of sheets and into a sacrificial strip- wood back then, usually plastic now. If the strip starts coming apart, or if some debris gets on the strip and remains between the table and the stack of sheets the pressure from the plate that holds down the stack can bend the sheets leading to a cut that isn't straight.
Usually there's a bit of creasing near the spot where the cut changes.
Here's a card with just that sort of cut. And light creasing right at the change.
Steve B