The numbers are not match in 7.1.2 - Mailing list pgsql-sql

When I do

select * from pg_database;

I got

datname     | ........... | datlastsysoid
------------------------------------
mydbname | ........... | 18539

But When I

cd $PGDATA/data/base/
ls -la

I got
1
18719
18725

Where is 18539 ??!!

I think mydbname is 18725
When I

cd 18725
ls

There are a lot of numbers. How do I figure out which number is which
table_name?

BTW, what is/are the advantage(s) used the numbers instead real text
name like
7.0.x used to do?

--Raymond