Two answers can look identical on the surface and differ completely underneath. On the surface sit the things you can see: fluent sentences, specific details, a confident tone. Underneath sits the only thing that decides whether the answer is a hallucination — whether the claim matches reality.
A claim is plausible when it fits what you would expect. A book title in an author's usual style is plausible. A date that falls in the right decade is plausible. Plausibility is a judgment about the shape of the claim, and it is exactly what fluent writing produces.
Truth is a separate question. It asks whether the claim matches the world: does that book exist, did that event happen, is that number correct. Nothing about the shape of a sentence can answer that. You have to check outside the answer itself.
This is why confidence is not a sign of correctness. The confident tone is part of the surface — it is how the reply is written, not a measurement of how likely it is to be right. An AI can be just as fluent about a fact it has right as about one it has wrong. The two columns below show the same reply split into what you can see and what actually decides the matter.