No idea. Maybe more vacuum? Psychological? Dumb luck?Why would the two spray different?
That is why I use the Sata RPS cups. They have a positive vent.I use pps and have for 10+ years. I will try the aluminum cup one of these days to see. I think I have a good lid for the cups.
I know when the pps get low they start to choke the gun. I have always wondered if they were slowing the gun because there is no vent.
If I used my sata's more I would switch to the rps cups.