A site ranks for hard terms in one topic but nothing in another because topical authority is topic-specific, not a single global score, and it does not transfer across unrelated subjects. In the first topic, the site earned its position the hard way: it built deep coverage of the subject’s questions, attracted links from sources in that field, and accumulated the trust that lets it compete for difficult queries. In the second topic, none of that exists. To a search engine evaluating that subject, the site is a stranger with no track record, so the strength in the first area simply does not apply.
The clearest way to understand this is to stop thinking of authority as one number attached to your domain. It behaves more like a separate reputation in each subject area. The coverage you built, the links you earned, and the signals that proved you know the first topic are all bound to that topic. They tell the engine you are a credible source there, and they say nothing about your credibility somewhere else. When you reach into an unrelated area, you start that reputation from scratch, which is why a site that crushes hard terms in one lane can be invisible in another.
This is why spillover does not happen the way people expect. A strong domain entering a new, unrelated topic often assumes its existing authority will carry the new pages, but the engine evaluates the new topic on its own coverage and its own links. Without dedicated content depth and relevant trust in the second subject, even a well-known site competes as a newcomer there. The authority is real, but it lives in the wrong place to help.
There is a nuance worth keeping honest: when the second topic is genuinely adjacent to the first, some relevance can carry over, because the engine sees an overlap in subject matter. But for truly unrelated topics, the gap is real, and treating it as a transfer problem only delays the actual work.
So when you see your site dominating one topic and floundering in another, redirect the effort rather than waiting for spillover. Build dedicated coverage in the weak topic, answer its real questions, and earn links relevant to that subject, exactly as you did for the topic you already own. Treat each topic as its own reputation to build, not one score to spread.