pg_stat_activity versus ps

I ran a home-grown self continuous stress test tool against my 7.4.2
database.

I banged 'ps' (running freebsd) while it was active and witnessed
several of the 'back end' postgres processes exeucting queries, commits,
inserts, etc....(the actual work the processes were doing was listed in
the 'ps' output)
however, if I checked pg_stat_activity during the same time period, I
saw nothing populated in the current_query column...ever.

Am I misunderstanding what the pg_stat_activity table is for?  I
confirmed that the procpid listed in pg_stat_activity was the same as
listed in my 'ps' process list....


Jeff