I've always noticed this as well. I used RY4 prefilled cartos from Smokeless Image, and refilled using their RY4 juice. No matter how much i filled or overfilled, the prefills ALWAYS last longer. Using syringe, condom method, or just topping off till it poured out the bottom and puddled on the top, they still didn't last as long. I think it's a matter of letting them sit for a few days to really soak into the filler, then topping off again.