They spent more than the $870 some billion in the original package. The money not spent yet is slated for other things and instead of taking money from unecessary stuff to extend cash for clunkers, the home tax credit and unemployemt, they just tacked the extra on top.
Spent where?
As of November of 2009, only $100 billion of that $787 billion stimulus was actually spent with another $90 billion of stimulus coming in the form of tax reductions = $190 billion.
http://cbo.gov/ftpdocs/106xx/doc10682/11-30-ARRA.pdf