Corporate profits are cyclical, running up during booms and down during busts. The recent recovery in corporate profits has been influenced by Fed expansionary monetary policy.
National income varies much less. So as a percent of national income, profit vary quite a bit.
As the charts show, in Q3 2012, national income was $13,912 billion and corp. profits were $1,752 billion. So corp. profits were 12.6% of national income.