select null + 0 question

Why is it that "select null + 1" gives null but "select sum(a) from
table" where there are null entries returns an integer?

Shouldn't the sum() and "+" operators behave the same?

TAL=# select null + 0;
?column?
----------

(1 row)

TAL=# select * from a;
a
---

1
(3 rows)

TAL=# select sum(a) from a;
sum
-----
1
(1 row)

Thanks,

Jean-Christian Imbeault