The Hindu doctrine of atman and the Buddhist doctrine of anatta are built out of the same analysis and reach opposite ends, which is why the atlas carries both and why they are best read together.
Both begin by taking experience apart (body, sensation, perception, mental formations, consciousness), and asking whether any of it is the permanent self. The Upanishadic sage answers that behind and beneath all of it stands the atman, the witness that is never itself an object, unborn and undying, and in Advaita identical with Brahman. The Buddha, running the same subtraction, answers that when everything impermanent is set aside nothing permanent is left over: there is no atman to find, only the process itself, and the search for a self within it is the error.
The disagreement is exact. It is not that the two traditions describe different things; they describe the same act of introspection and disagree about what it discloses.
This is one of the sharpest doctrinal boundaries in Indian religion, and it is a genuine philosophical dispute rather than a difference of emphasis.