To me, this says that the directory does an internal group search to generate the isMemberOf attribute on the fly. I believe this is the way Active Directory handles the memberOf attribute as well.
IIRC in AD memberOf is a linked attribute, stored permanently.
From memory, AD stores the entry IDs of the link tuples in a separate table, so both "member" and "memberOf" are ostensibly generated on the fly. But this is really an implementation decision (although things do start to get interesting when dealing with references across partition boundaries).
-- Luke