Game progress can live in three places: the console or PC's local storage, the platform's cloud save service, and sometimes the publisher's own account tied to a separate login.
Conflicts arise when those disagree. Playing offline on one machine and then syncing can overwrite newer progress with older, and most systems ask a question that people click through quickly.
Cloud saves are also usually a subscription feature. When the subscription lapses, uploads stop silently and the cloud copy quietly becomes months out of date.
For games with a publisher account, linking that account is what actually protects progress across hardware changes — the platform save alone will not transfer.