order of adding date & interval values?

hello,

using 7.2.1

is there any reason why the order of operations of the following query
would matter?

here's an example:

diw=# select interval '40 years' + date '2001-01-01' as test;
test
---------------------
2001-01-01 00:00:00
(1 row)

diw=# select date '2001-01-01' + interval '40 years' as test;
test
---------------------
2041-01-01 00:00:00
(1 row)

note how the first query doesn't return the proper response.

also, is there a difference between:
"interval('40 years') " and "interval '40 years' " ?

or

"date('2001-01-01')" and "date '2001-01-01' " ?

because only the query at the top of this message works, the following
doesn't:

diw=# select date('2001-01-01') + interval('40 years') as test;
ERROR: parser: parse error at or near "'"

I realize that answers my question, but why?

any help would be appreciated!!!