performance question

Can somebody explain to me:

> radius=# explain select count (radiuspk) from radius ;
> NOTICE:  QUERY PLAN:
>
> Aggregate  (cost=12839.79..12839.79 rows=1 width=8)
>   ->  Seq Scan on radius  (cost=0.00..11843.43 rows=398543 width=8)
>
> EXPLAIN


This query answers me *instantly* after hitting return

> radius=# select count (radiuspk) from radius ;
>  count
>  --------
>   398543
>  (1 row)

This query takes about 3 seconds. But the query plan *already* knows the 
number of rows ("rows=398543"). So why does it take 3 seconds. Is my 
assumption correct that the optimiser still can be optimized a little? :-)

Reinoud (not that this is a real problem, just wondering)