Minimum droplet size for both printers is 3.5 picoliter, I doubt that the 7900 improved weaving/dithering results in a higher sharpness.
The smart print sharpening setting in applications like Lightroom or Qimage Ultimate can be overdone. I guess some RIPs will have that feature too and can be misused.
It would be worse and against your request if there is also an extra sharpening done by the print shop before the printing stage.
If none of both was applied (though some smart print sharpening should in theory) then you possibly see better upsampling routines in the print shop's print. That and selecting Fine Detail in the driver so the driver asks for 720 PPI may make a difference + the slower print speed then which should create less dot gain. I am not familiar with the 7900 results to say that it should make that difference. My bet is the smart print sharpening overdone.
Met vriendelijke groet, Ernst
Dinkla Grafische Techniek