Yes, the non-union employees will end up taking more home (paid the same but like you say less goes back to the Union, Insurance, Retirement, Training) than their union counter parts (assuming the boss is honest), but the idea is the Contractors need to base their bids off that number as to make the bids fair to all contractors both Union and Non-Union. Guessing the government has some auditing to ensure the employees are paid according to the Union wage of that location/trade. Without a prevailing wage would be very difficult for Union shops to get work on some of these Government projects.