Why does numeric_out produce so many trailing zeros?

I've been reading the documents on numeric and I can't find any information
on the reason that a query like this:

test=# select n::numeric / 1 from generate_series(1,2) s(n);
?column?
------------------------
1.00000000000000000000
2.0000000000000000
(2 rows)

produces results that have so many trailing zeros. Also I'm wondering why
the first row has 20 trailing zeros and the 2nd row has just 16?

Is there any reason that we output any trailing zeros at all?

Regards

David Rowley