Kevin Montgomery, writing for Wired:

Currently, Google allows advertisers to target their ads based on gender. That means it’s possible for an advertiser promoting high-paying job listings to directly target men. However, Google’s algorithm may have also determined that men are more relevant for the position and made the decision on its own. And then there’s the possibility that user behavior taught Google to serve ads in this manner. It’s impossible to know if one party here is to blame or if it’s a combination of account targeting from all sources at play.