"About this Mac" not showing correct local backup size
TimeMachine uses hard links to do his magic for incremental backups. so if you do a du check on a TimeMachine backup volume, you won't see good results, because hardlinked files would count twice or more but in fact they use just one. if you check for disk usage, please exclude any form of hard links (backup directories). there is no option to "exclude hardlinks" similar to not follow symlinks, because the backup helper cannot distinguish between a hardlink and a real file.