So what @maiki said here is correct and is actually the behavior that we want when search results are ordered by relevance. However, we had a bug in our code previously where we would only show either the earliest post or latest post of a topic that matches the given term.
This can still be achieved with the in:first filter.
Something definitely seems to be off with the search results here because I can definitely find other posts which I think should be ranked higher. Let me investigate.