It seems plausible to me that some people might be discouraged by thinking of stereotypes, and others might get angry and do better, and once you average it out

It seems plausible to me that some people might be discouraged by thinking of stereotypes, and others might get angry and do better, and once you average it out, you don't find an effect. People often talk about how you can find an effect that doesn't exist by slicing and dicing your data, but you can also cover up one that does exist by not dividing it up.