They were definitely all white when they left the factory.
They could have been stored badly anywhere, even at the casino. Too much humidity can yellow them.
It wouldn’t necessarily happen to all of them either. Different batches of photographic paper, or a different brand even, will react differently to light, heat, humidity.
Some can eventually get stained yellow from the adhesive behind.
Sometimes it’s only the clear laminate on top that is stained (impossible to tell unless you can manage to peel one apart).
Many different variables.