Well....people come straight run and the percentage of the US is about 51 to 49 % in favor of women. Chickens are probably about the same, 50/50. But that doesn't mean out of 20 straight run chicks you will get 10 hens every time. You might get 19 roosters. Just like if you set 20 eggs you might. It's really luck of the draw, like tossing a coin 10 times---in theory you get 5 heads and 5 tails but in reality that doesn't happen often. If you want one sex or the other go for sexed chicks but even sexed chicks only have a 90% expected accuracy so you can expect one "mistake'" out of every 10.
I doubt hatcheries are going to mess with straight run---it takes too much time and the minute amount of money they save isn't worth the effort or the potential damage to customer relations.