2018/11/27: we know Mueller has the goods on whatever Manafort lied about -- and whoever Manafort felt a need to protect -- because he told the judge that he "will file a detailed sentencing submission to the Probation Department and the Court in advance of sentencing that sets forth the nature of the defendant's crimes and lies, including those after signing the plea agreement." Translation: Manafort lied, I know it, and I'll prove it at his sentencing if necessary. That should cause serious concern to whoever Manafort sought to protect by lying to federal prosecutors during his proffers.
So, why then would Manafort endanger himself by lying to Mueller? I see two possible explanations. One, perhaps Manafort is simply an arrogant, congenital liar. The trial evidence proved that Manafort believed he could spin a web of lies and get away with them. He got caught lying to banks and the IRS, and now, unsurprisingly, he has been caught by Mueller. Some cooperators understand that they are best served by telling the truth, for better or worse; others play games with prosecutors and end up getting burned.
Two, Manafort may be playing for a longshot pardon or commutation from Trump. It already has been reported, before Manafort's trial conviction and cooperation plea, that Trump at least discussed the possibility of pardoning Manafort.
2017/11/02: What if Mueller proves his case and it doesn't matter?