It was a difference in flavor. At the time, I attributed it to the decaffeination process that was used, thus causing the drink to taste different. It was a mild difference, but detectable. And I didn't like the different taste.

Thinking back on it, it could just have been that the caffeinated drinks came from a newer batch and the decaffeinated drinks came from an older batch. Or maybe they were canned in different facilities, which might be using different water.

I don't think my mom controlled for all the potentially confounding variables.