Unions Vital to Middle Class, But Their Workers Make Less

I listened to NPR's On Point today with rapt attention - a chance to get the straight liberal "unbiased" attack on Wisconsin. To establish balance, host Tom Ashbrook turned to a guy named Harley Skaiken, a labor professor at the University of California, Berkeley.

Shaiken's main points were that 1) unionized public sector workers in Wisconsin make about 5% less than their private sector counterparts and 2) that weakening the public sector union representation would destroy the middle class.

These two points seem contradictory to me. Can you figure it out? It would seem the best thing you can do for the middle class would be to get them out of the union.