Additional Details On Prosper's Default Sale

One of the advantages of tracking how loans age is that some interesting tid-bits appear. Prosper finished a just-in-time-for-tax-year debt sale at the end of December. While researching the loan aging during December, I extracted a few details about the debt sale.

Of the 702 loans that transitioned to default from Dec 1, 2007 to January 1, 2008, 672 were 4+ months late and 30 were 3 months late on Dec 1. What's more curious is that 142 loans that were "4+ Months Late" on Dec 1 did not go to debt sale. Now, we know from Prosper that there are 68 loans active in their legal collections effort. What, then, has held up the sale of the other 74 loans? This makes me wonder if there is some detail or factor that reduces the odds of debt sale that may be very relevant to lenders. Time for some investigation.