I am very, very confused as to how the upgrade process works exactly. I have read every Delta help topic and multiple threads on here and am still confused.

On a recent flight, as a lowly FO I was somewhere around 50/75 on a flight from ATL to STL for a Delta Comfort+ upgrade. However, when scanning my boarding pass at the gate to board the plane, a piece of paper pops out for a Comfort+ upgrade to seat 10D!

How in the world is that possible when I was around 50th on the upgrade list? It doesn't make any sense. Any explanations would be helpful!