The straight run bins usually do start out as 50/50 but are "picked over" by the first customers who are able to take educated guesses on gender and choose the pullets. I'm guilty of that myself! My 2010 Spring chicks came from a bin of straight run chicks BSL, GLW and SS. The BSL was easy, of course. The GLW and SS were a matter of finding the chicks with the smaller legs and feet. However it worked and I ended up with all girls. It is quite possible the same thing happened to you - earlier customers picked out the girls from the bunch before you got there so although still technically straight run, you had a much greater chance of getting the boys.