nextval() clarification

Long time MySQL user, six weeks into Postgres here.

I'm converting a PHP script that uses MySQL to use Postgres instead.
To substitue the missing MySQL functionaility with mysql_insert_id()
and auto_increment I am using code like this:

$sql = "SELECT nextval('companies_company_id_seq'::text)";

And then I do my insert with that value.

It works fine but I don't understand why it always gives me a +1 value
of what I'm expecting.  For example when I have an empty table it
gives me 2 as the first nextval instead of a 1, or if I have two rows
in there already it gives me a 4.  Is this by design or am I missing
something?  It's not a problem I just want to know what's going on.

TIA..

--
Greg Donald
http://destiney.com/