I think the question relates to the fact that "Used" pages are not always *actually* in use.
But it seems that this is not a very important question. The important thing to monitor is that the percentage of used does not get close to the max. But also, note that the maxsize can be larger than the underlying disk storage available space. This can be dangerous. It's always important to monitor both the used/max percentage as well as the used% of the underlying disk storage.
Now that we're on this topic. And this is only an academic question, probably: I'm wondering what are olmMDBPagesFree? It seems intuitive that (max - used) would equate to free, but it does not. Are "free" pages pages that were once used?
$ ldapsearch -ZH ldap://ldap2.rexpro.local:3892 -w $W -s base -b "cn=Database 1,cn=Databases,cn=Monitor" olmMDBPagesMax olmMDBPagesUsed olmMDBPagesFree dn: cn=Database 1,cn=Databases,cn=Monitor olmMDBPagesMax: 262144 olmMDBPagesUsed: 34465 olmMDBPagesFree: 395