I'm not sure why there is a reason for such behaviour.
select count(distinct id) from bg order by id;
ERROR: column "bg.id" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: select count(distinct id) from bg order by id;
Quite apart from the fact that you're trying to ORDER a recordset that contains a single row (why?), in Postgres (unlike MySQL) you can't order a list of values by a column you haven't selected.
Is this what you're trying to achieve:
SELECT COUNT(*), id FROM bg GROUP BY id ORDER BY id;