The value of the onReorderCallback's newIndex and oldIndex parameters are non-intuitive due to some now-historical flawed API engineering. They're explained here: https://api.flutter.dev/flutter/widgets/ReorderCallback.html, but not in https://api.flutter.dev/flutter/material/ReorderableListView-class.html or in https://api.flutter.dev/flutter/material/ReorderableListView/onReorder.html.

We decided not to fix the problem because there wasn't a way to automatically update old apps: #93146 (comment) which many developers were unhappy about.

The documentation for the ReorderableListView and for ReorderableListView.onReorder should explain this quirk.