ARRAY(subquery) volatility

Why does the first query below return the same value for each row
while the second query returns random values? Planner optimization?

test=> SELECT ARRAY(SELECT random()) FROM generate_series(1, 5);
?column?
---------------------
{0.269273371561092}
{0.269273371561092}
{0.269273371561092}
{0.269273371561092}
{0.269273371561092}
(5 rows)

test=> SELECT ARRAY(SELECT random() + x * 0) FROM generate_series(1, 5) AS g(x);
?column?
---------------------
{0.826863945846848}
{0.42534113182935}
{0.36419924318986}
{0.258920902972538}
{0.843205466327819}
(5 rows)

--
Michael Fuhr