Tracking database activity

Hi, I'm hoping someone can help me wrap my head around some #s I'm using
to track database activity.  I have a script that runs hourly and
queries pg_stat_database and checks age(datfrozenxid) in pg_database.
It logs those stats and the next hour, when it runs, it takes the
differences to check the amount of activity over the last hour and then
logs the new stats.  I was previously under the impression that by
tracking age(datfrozenxid) in pg_database over time I'd be able to know
how many transactions were processed in a certain amount of time.
However, I've seend that pg_stat_database.xact_commit +
pg_stat_database.xact_rollback < pg_database.age(datfrozenxid) by a
factor of as much as 6 for any given time frame.  Am I misunderstanding
something here?  Where is the discrepancy coming from?

--
erik jones <erik@myemma.com>
software development
emma(r)