sum() over (partition by order) question

Hello

I am play with windows function. I was surprised so these queries has
different results.

postgres=# select sum(a) over (partition by b), a, b from foo;sum | a  | b
-----+----+--- 19 |  1 | 1 19 |  1 | 1 19 |  2 | 1 19 |  4 | 1 19 |  2 | 1 19 |  4 | 1 19 |  5 | 1 93 | 11 | 3 93 | 12
|3 93 | 22 | 3 93 | 16 | 3 93 | 16 | 3 93 | 16 | 3
 
(13 rows)

postgres=# select sum(a) over (partition by b order by a), a, b from foo;sum | a  | b
-----+----+---  2 |  1 | 1  2 |  1 | 1  6 |  2 | 1  6 |  2 | 1 14 |  4 | 1 14 |  4 | 1 19 |  5 | 1 11 | 11 | 3 23 | 12
|3 71 | 16 | 3 71 | 16 | 3 71 | 16 | 3 93 | 22 | 3
 
(13 rows)

I didn't expect so ORDER can change result of function sum. Please,
can somebody explain it?

regards
Pavel Stehule