Relations summary at end of vacuum

Hi,
What exactly are the "Relations" that are referred to in the summary
at the end of a vacuum? I have two DBs that are supposed to be mirrored
copies of each other, but my relation & page counts are different for
each after a vacuum:

DB_a:
INFO: free space map: 109 relations, 70413 pages stored; 71696 total
pages needed
DETAIL: Allocated FSM size: 1000 relations + 100000 pages = 647 kB
shared memory.

DB_b:
INFO: free space map: 281 relations, 57954 pages stored; 61904 total
pages needed
DETAIL: Allocated FSM size: 1000 relations + 100000 pages = 647 kB
shared memory.

The schemas and DBs are set up exactly the same, so I'm rather confused
as to the discrepancy. Data checks (sums on key columns, etc) show that
the data does appear to be in sync. I checked pg_tables, pg_indexes, and
both have the same info.

If this is normal behavior, I'd just like some additional info to set my
mind at ease.

Thanks