Sexing a baby chick is not an exact science. No two chicks are identical. We use three sexing methods: color, feather, and vent. The method we use depends on the breed. The chicks are hatched and sexed the same day which means that they are very small and this sometimes leads to mistakes in what we are looking at. When you combine the sexing mistakes with human error and the 3 methods not being an exact science you are running a risk of getting a rooster (cockerel chick). The best we can do on an overall average of gender sexing is 90% accuracy.
For more information and to report a mis-sexing:
https://help.chickensforbackyards.com/hc/en-us/articles/1500006314922