Difference between Diet Coke and 7UP

Diet 7Up contains aspartame - but it gives me no buzz. Why not?

http://dir.salon.com/story/health/feature/1999/09/30/diet_coke/index1.html

"I have a special request," I began. "Could we get some diet Sprite or diet 7-Up in our next shipment?" (Neither of those contain phosphoric acid or caffeine.) "No problem," she said."