I think the norm is 90% correct sexing. The hatcheries don't guarantee sexing 100%... I'm assuming they had gotten a big batch of chicks in at the feed store?... Maybe you wound up getting the unfavorable 10%
Also, like some body else stated, there are a lot of people handling the chicks, and they can easily get mixed up. Best thing to do if you want to be absolutely sure about sex is to get them once they are old enough to tell for sure, other wise you'll either be eating chicken dinner or selling a few roo's every now and then. I've never gotten any chicks sexed or not, with wing bands... Not from a hatchery or a breeder? But, there again, you'll still have that 10% chance... a band doesn't mean much in that respect. Obviously no offense intended
Also, like some body else stated, there are a lot of people handling the chicks, and they can easily get mixed up. Best thing to do if you want to be absolutely sure about sex is to get them once they are old enough to tell for sure, other wise you'll either be eating chicken dinner or selling a few roo's every now and then. I've never gotten any chicks sexed or not, with wing bands... Not from a hatchery or a breeder? But, there again, you'll still have that 10% chance... a band doesn't mean much in that respect. Obviously no offense intended