http://www.housingwire.com/2008/04/10/a-tale-of-two-markets-housing-price-risk-diverges-during-fourth-quarter/
‘Berson said he expects the S&P/Case-Shiller housing price index to fall 20 percent from peak-to-trough, translating to an 8 percent decline in the OFHEO housing price index. Both indexes are widely watched by market participants, but measure different aspects of the U.S. housing market.’

Can someone explain why these two indeces are so different . They use the same methodology, though I know CS is 20 cities only.
Thanks. Graham