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