Centrist version of liberalism?

The New York Times ran an interesting piece today about interns at the Heritage Foundation. There was a very strange statement when other programs were mentioned.

The Brookings Institution, a centrist group more than 50 years older than Heritage, has no paid interns. Neither does the Progressive Policy Institute, which promotes a centrist version of liberalism. The Center on Budget and Policy Priorities, a premier antipoverty group, has 10 paid interns. People for the American Way, a bulwark of Beltway liberalism, has 40 - but no dorm.

What the heck is a centrist version of liberalism? Why can't they just call the PPI what they are, liberal?