If you're implying that HN would generally prefer more female coworkers and don't really prefer to have more black ones, you may be on to something obvious :)

I guess the above explanation is too uncharitable, and people don't believe they're racist but do believe they (or the industry) are sexist. I feel like racism is somehow more evil. I think "low level racism" is a decent label - you shouldn't be surprised that at a superficial level of acquaintance (resume screening), people subconsciously pull expectations of competence/"fit" according to the reference class they see the resume as belonging to (oh, another 25 yr old brogrammer!)

Also, there are more women in the U.S., so if women and blacks were similarly discouraged from coding, we'd want to throw more resources at encouraging women, hypothetically.