Why Mubarak Leaked His "Resignation"

The rumors that Mubarak was going to step down today were widely heard across the world.  Protesters listened in quiet anticipation to hear the words they had been fighting so hard to hear...but 12 1/2 minutes into Hosni Mubarak's speech, all of their built up anticipation had turned to anger.  They realized he wasn't resigning, and the anti-Mubarak chants erupted into deafening roars.

What purpose was there to build everyone up to a resolution to the revolution only to bring them crashing down?  Did Mubarak just decide backstage he had changed his mind?