Saw this all the time with my (horrible) Concur internal corporate travel system. BS was usually a few dollars cheaper. I believe they must extend a 5% BS discount to Concur. Do the math and see if that is it. I did take full advantage of it. The sweetest deal for me was during the WN-Airtran interim where BS would get you first class on Airtran. WN points and great service/seating, best of both worlds.