The reason is because income was up slightly higher than the .3% growth analysts were expecting.
In the end, income gains and higher savings rates means a base is building for consumer to borrow multiples of that. The demand for loans is what is causing the Australian Economy to percolate (however transiently). America's swalled it's medicine, and although long term factors, specifically education don't bode well for America, other countries have been making the same mistakes America made before its demise and pretty soon many other countries will find themselves overborrowed and without a reserve currency status to print their way out of the mess.
That means declincing tax revenue, budget deficits, mortgate blowups, bank failures, and currencies forced downward due to the inevitability that those countries will bail themselves out with easy money.
Small victories like today's income gains for Americans may be the beginning of a much larger story. A story where America may find some stability (probably not dynamic growth) while many countries around the world (specifically Australia) finds themselves in a larger and more protracted version of the same thing America has gone through over the last 5 years or so.