Hatcheries do vent sex their Easter Eggers, but the accuracy rate is usually closer to 75% rather than the claimed 90%. Regardless, when buying sexed chicks, there is always a chance for an accidental cockerel or two.
I think they can vent at 90%... but prefer to ship at 70%...
Last year I ordered 100 hens CX's for eating. I got 100 hens. I order 50 roosters the year before and got 10 hens. (hens having less value as broilers.)
The year before that I ordered 50 straight run and got 41 hens. Color me skeptical but.. the numbers suggest someone plays loose and fast with the 90% sexing rate.