I wouldn't say it's about keyword density per se, but more about keyword relevance. They have 10 blog posts highly optimized and targeted toward a particular keyword. Presumably the content of those posts is relevant and highly focused on the topic. Do you have a similar amount of content that is similarly tightly focused? It sounds like it's a matter of their singular focus on that one topic and the exclusion of other topics from the section that, all else being equal, is giving them an edge.