Number of tuples in pg_class - bug or misunderstanding?

I'm just digging a little bit in PostgreSQL internals and I found the
following thing. Shouldn't the reltuples field in pg_class contain the
real number of tuples? It is always a little bit to low ...

irc=# SELECT reltuples FROM pg_class WHERE relname = 'events';
 reltuples
-----------
       188
(1 row)

irc=# SELECT COUNT(*) FROM events;
 count
-------
   195
(1 row)


This annoys me a little bit as I can do
  SELECT reltuples FROM pg_class WHERE relname = $1;
from plpgsql, but not
  SELECT COUNT(*) FROM $1;
--
Martin Neumann
Appartement 201, Emil-Figge-Str. 9, 44227 Dortmund, Germany
mne@mne.de - http://www.mne.de/ - PGP key @ http://www.mne.de/mne/pgp.txt