I also suspect that there is a certain personal dynamic that goes on.  If your a private lawyer, you walk into court representing one client, you can argue, look, this guy is different from everyone else on the calendar and hears why.  And if you happen to know, or especially if you're friends with the DA (it's a small world afterall) then, consciously or not, there's a likelihood of a bit of a favor being done.   But even if the DA likes the PD, he/she can't do a favor to 80% of the cases on the calendar.  And as a PD, it's hard to argue "this guy deserves a break" without implying, "the other 10 clients I have with the same charges and history don't deserve a break."
PD clients tend to get rubberstamp disposition offers.  It's easier for a private attorney to distinguish their client and to help craft a more individualized and rehabilitory (I may have made that word up) sentence.