Any extra sexed chicks will be used to substitute for the same sex of another breed that had a bad hatch that week (it does sometimes happen that they don't all hatch like we expect them to), or to fulfill assortments. Sometimes there are birds that the sexer is not sure of, too. It is an art, not a science, after all! Those the sexer is not sure if will go in the straight run bin because we want to do our absolute best to give you the sexes you order. Even still there are rare mistakes, but at least you are covered by our 100% sexing accuracy guarantee !
That being said, straight run babies are shipped as hatched. That means, if they hatch with an out of whack ratio, you receive an out of whack ratio. Because they are unsexed, we have no way of knowing what sort of mix you will get. Unfortunately, it is thought that pullets are preferentially killed by variations in incubation temperature, while males are preferentially killed by cooler storage temperatures before incubation... so getting off ratios does happen. Frequently. We wish we could control the number of males and females hatched out as much as you do!
With large fowl, we think it pays to go ahead and order the sexes you want. It isn't that much more expensive for a guarantee of pullets/ cockerels. Sexed bantams are more expensive, but if you are ordering just a few, having an off ratio can seriously affect your flock. For instance, if you order 5 straight run bantams, you would normally expect to get two or three cockerels, based on a 50-50 average. But if you got just ONE more cockerel than the average in your straight run order, that would put you at three or four cockerels out of five. Only one hen. And that would be--pardon my French--sucky. Very, very sucky.
If you were to order, say, 50 straight run birds, having a male-female ratio that is very askew is less likely to happen so straight run might be something to consider with a larger order. Maybe. But it's not something we would recommend.
We get questions related to this topic all the time, so perhaps addressing it again will help convince you. (Read the first answer here.) We'll address it a third time, as needed. :)
The bottom line is that if you want a small backyard flock, unless you're a big risk taker, and unless you're willing to deal with the possible disappointment of getting fewer hens than you wanted, then you'll probably be much happier receiving sexed birds.