Straight run birds are an "as hatched" mix of males and females. Therefore, if you were to order eight straight run birds, for example, you will get between zero and eight roosters and between zero and eight hens--there is no way to tell.
Straight run chicks are sent "as hatched"--they are not sexed first and then separated out into each order. The reason is that expert sexers of day old chicks get top dollar since it is such a specialized skill--this is why ordering all straight run birds is less expensive than ordering all females. The only way to get sexed baby chicks is to purchase them sexed.
Generally the hatching ratio of straight run orders averages out to be a 50-50 mix, but you are more likely to hit the average if you have ordered large numbers. For instance, you would expect that if you flipped a coin 1000 times, you would probably get heads around 500 times. Even if you had a few runs of flipping tails all in a row, things would be equalled out eventually by runs of heads.
If you flipped a coin only ten times, though, there is no guarantee you would get exactly five heads and five tails. You might get four and six, seven and three, eight and two or nine and one. It is even possible to get all heads or all tails, though it's not terribly likely.
As an example, I once hatched 8 eggs at home, and seven of them turned out to be roosters (which is a terrible laying flock)~(:>3)=. The next time I hatched eggs, I got three hens out of three eggs when I was hoping for at least one rooster of that breed. Most recently, it looks as if I got two roosters and one hen out of three hatched. So, the first time I got 88 percent roosters, then zero percent, then 66%.
You should only order straight run when you are flexible and will be happy with any ratio. For myself, I'm not much of an odds player, so when I purchase birds from a hatchery, I always get them sexed.