I think it's down to the fact that when I tighten the out posts, the dip tube moves along the bottom until it touches the base. How much open area is left dictates the flow rate.
I had bought 3/8 JG in line valves to stop the corny's at fill volume, but now I just turn the fv tap off, detach the out connector from that corny, then open the fv tap and so on.