Your husband is putting some money in Retirement accounts, possible medical flex accounts, or other types of accounts that are reducing the wages being taxed (box 1 of the W-2). 

Look at the W-2, and in box 12 is where most of these should be.  The big one is probably a code "D" a 401K plan, which is excellent to do, as it saves for retirement and also reduces taxes paid now.  

Post a comment if there is a code you are not sure about.  Thank you.