There are actually 75% less flats built between 2003-2007 than from 1998-2002. The 2003-2007 batch is up for resale now, taking into account the 2 year construction phase and the 5 year MOP. Basically there is a supply crunch...
...which will not be eased till 2017 when the huge 2010 batch of BTOs are finally available on the resale market
Demand though weaker is still there thanks to the core of PRs, downgraders and first-timers who still want to live in an estate of their choosing.
More HDB upgraders are keeping their flats too, renting them out to pay for their mortgage of their private units.
HDB failed to anticipate economy recovering, and ramped up the supply too late and too slow, 5500 BTO flats in 2007, 7800 in 2008, 9000 in 2009 (global recession), 12000? 16000 in 2010, 22000 25000 in 2011, 25000 27000 in 2012, plus some DBSS and EC since 2010, but still not enough to not cope the demand. (striketrough numbers are initial numbers announced at beginning of year).