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