Things such as a college degree and experience are obvious signals, but they are not the only signals. Race and sex play a part as well, assuming that different races/sexes exhibit different behavior on average. This is statistical discrimination[1], not bigotry. In fact, this discrimination is efficient because it lessens the likelihood of choosing a bad employee. This is why men are given more job offers than women- a man is less likely to go on paternal leave or switch to part time because of 'stress'. If this discrimination did not occur, the employees would be of lower quality which hurts productivity.
[1] http://en.wikipedia.org/wiki/Statistical_discrimination_(eco...