A long-tail page outranks a head-term page when it matches a specific intent more precisely and faces weaker competition, so for that exact narrow query it is simply the better answer. The broad head-term page tries to cover everything and ends up too general to fully satisfy a specific question, while the long-tail page is built around that one question and answers it cleanly. Specificity plus lower competition beats breadth for the specific query, and this shows up often enough in real rankings to be worth understanding rather than treating as a fluke.
The mechanism is precision of match. When someone searches a detailed, specific query, Google looks for the page that addresses that exact need, not the page that mentions the topic in passing. A focused long-tail page devotes its whole structure, every heading and example, to that narrow intent, so it reads as the most relevant result. The head-term page covers the same ground in one section among many, diluting its relevance to any single specific question. For the narrow query, the focused page is more on-target, and relevance to the actual search is what ranking rewards.
Competition is the other half. Head terms are contested by the strongest, most authoritative sites in the space, so a broad page is fighting uphill against established domains. The long-tail query draws far fewer competitors, often pages that treat it as an afterthought, which leaves room for a dedicated page to win even from a smaller site. So the upset is not mysterious: the long-tail page won because it matched the intent better and had less to beat, not because head terms suddenly lost their value on broad searches.
Take this as direction for how you build. Instead of pouring everything into one broad page and hoping it covers every angle, create focused long-tail pages for the specific intents your audience actually searches. Each one can match a precise need and slip past the heavy competition the head term attracts, giving you ranking entry points the broad page alone would never reach.