This happens when you use drag and drop to extract files or open archived files with an external viewer. In such cases files may still be in use when closing WinRAR and moreover, there is no reliable way to detect if the external program still needs these unpacked files. So WinRAR does not delete them immediately. Instead, when you run WinRAR the next time, all temporary files older than one hour will be deleted.

If you do not want to involve the temporary folder when extracting files at all, use either the "Extract To" command in the WinRAR shell or the "Extract To" item in the archive context menu instead of drag and drop.