Odd behaviour -- Index scan vs. seq. scan

I can't find a reasonable explanation for this.

I have a table game, with primary key gameid (an int).

If I use a where involving gameid and <, or >, or <=,
or >=, then I get a sequential scan.  If I use =, then
of course I get an Index scan.

More surprising to me is the fact that using BETWEEN,
I get an Index scan!!

The following is a copy-n-paste of the EXPLAIN outputs
(with the useless lines removed):

     explain delete from game where gameid = 1000;
     Index Scan using game_pkey on game  (cost=0.00..3.14 rows=1 width=6)

     explain delete from game where gameid < 1000;
     Seq Scan on game  (cost=0.00..4779.50 rows=200420 width=6)

     explain delete from game where gameid between 1000 and 2000;
     Index Scan using game_pkey on game  (cost=0.00..3.15 rows=1 width=6)


How's that possible?  Is it purposely done like this, or
is it a bug?  (BTW, Postgres version is 7.2.3)

Thanks,

Carlos
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