The 100bhp JTD that was available in the Stilo was not multijet, only unijet. So it's not the same as in the Punto HGT Multijet.
I believe it has been removed from the Stilo range now (along with the 115bhp JTD), and has been replaced by the 120bhp multijet, most likely because of the euro4 norm.
But the difference between the 80bhp and the 100bhp JTD's could be something as simple as a different ECU.