Very true. My first batch of chicks from TSC was 8 hens, 1 cockerel. Second from a hatchery. 3rd, from TSC, 4 straight-run bantams, 2 were cockerels, one pullet was standard-sized. A relative got 1 cockerel and seven hens from TSC. My batch from a hatchery was all hens.
My theory is that Hoover's Hatchery, where most TSCs get their chicks, will refund if you get an accidental cockerel, but if they ship to TSC, accidental cockerels aren't their problem, and therefore they are less careful with sexing. Not sure if this is true, but it is a possibility.