Global Filters are used to exclude content from backups that is "useless" for example temporary or log files that you wouldn't want to restore or that get replaced upon reboot. (Think what would happen if you backed up the swap file on a Windows machine).
Excluding this unwanted data can speed up your backups and obviously make them "smaller"
As far as the data aging goes I'm almost certain you have default settings (Simply check in the "Scheduler" window and filter it if need be).
The problem I am trying to address is simply that a day is based on 24 hours (yes I know that sounds silly:).
If a backup just meets retention at 12:00 noon time when the data aging runs, the old data will still be kept. If you schedule another right before your backups it will be pruned off, thus giving you the maximum storage available.
The easy way of scheduling another data aging is to right click on the Commserve entry on the top of the browser tree and select data aging, you can then set up a schedule in the Wizzard.
Let me know if this clarifies.