There is an obvious psychological explanation for these results, so obvious that it would be shocking if this preliminary study had shown anything different.

That explanation is that men fear the disapprobation of other men far more than they fear it of women, while women fear the disapprobation of everyone equally. So the male baristas are not discriminating against the women so much as they're discriminating in favor of the men, to minimize the chance of any unpleasantness from male customers. The psychological "cost" of ignoring a woman is lower than for ignoring a man. That's why the discrimination gets worse with busier stores and male staffs. In a busier store, the chances of customer dissatisfaction grows, so the effort to prevent it in the male customers increases. It also explains the less pronounced age discrimination--older customers are less patient and more outspoken.

And indeed, we see the signs of such discrimination in some of the responses in this thread. Misogyny generally manifests itself not so much in outright hatred as militant apathy. Men are tired of women complaining, when they do complain, and men are tired of taking the blame for "women's problems", except they're not the women's problems, they're very much the men's problems.