LV volume space always loses 33M?

[Gabriel M Dragffy]

Sorry for yet another post on LVs, but I'm justing get comfortable with
LVM.
One other thing that troubles me is a "df -h" report. Cutting out a lot
of other info from it, this is an example output:

/dev/mapper/bouncyball-optional
                      300M   33M  268M  11% /opt


This volume is empty but 33M has disappeared. It would seem all volumes
"lose" 33M... where and why? Can this be changed?