numeric cast oddity

When I cast an integer to numeric using :: notation it ignores the scale
and precision that I specify, but when I use the cast function it uses
the scale and precision that I specify.

Sim

select version();
"PostgreSQL 8.3.5 on i586-pc-linux-gnu, compiled by GCC
i586-pc-linux-gnu-gcc (GCC) 4.1.2 (Gentoo 4.1.2 p1.0.1)"

select -1::numeric(20,4)
?column?
numeric
------------
-1.0000

select cast(-1 as numeric(20,4))
numeric
numeric(20,4)
-------------------
-1.0000