If you installed assets previously, and then a bundle removed its Resources/public (or you removed the bundle entirely), the corresponding bundle folder in web/bundles is not cleaned when running assets:install again.
Getting a clean web/bundles folder requires deleting the folder and running the install again.

Note that you even end up with broken symlinks in web/bundles in case assets were symlinked rather than copied.

Originally reported as symfony/flex#22 for a case impacting the removal of a bundle.
See symfony/demo#588 for such a case about removing public assets from existing bundles too (we missed at least 2 removals in the history, as the sensiodistribution folder was unused since before the demo switched to symlinks, while framework was after).