My thoughts are that there are possibly a number of factors:
1. Different number/size of perforations between the baskets.
2. Same tamp pressure on a deeper basket (presuming the smaller diameter basket is deeper) may result in a less compacted puck.
3. 9bar at OPV may not be 9 bar at shower screen.
4. Not sure how the pump functions exactly, but perhaps the flowrate is limited?
My money is on point number 2.