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;

reltuples is updated only by VACUUM. The performance hit is too great
to keep it up-to-date, we think, though if it was accurate, we could
optimize COUNT(*) with no WHERE.

-- 
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