A variety of possible reasons, depending on how you have set up the two systems:
*If you use the 5.1 analog outputs on the 3910, then speaker levels, delays, and subwoofer crossover point are being set by the 3910, not the 3805. Maybe you have them set up better on the 3910 than on the 3805.
*Correct me if I'm wrong, somebody, but I think the RoomEQ circuitry is not being used on the EXT IN inputs...
*All other things being equal, the 3910 has better DAC's than the 3805. It's the 3910's DACs that are in use when you use the EXT IN connections, and the 3805's when you use DLINK3 or any of the other digital cables to connect these pieces of equipment.
I found that if I set the speakers and bass management the same on both the 3805 and the 3910, and if I turn off RoomEQ, then EXT IN gives me better sound (especially better imaging and a deeper soundstage) than using any of the digital connections. Others report that being able to use RoomEQ on the digital connections has given them better results than analog connections, even if (on paper) the 3910 has better DAC's. Let your ears deliver the verdict!