'isNull(Field1,0)' Should Return 0,but It Didn't, WHY?

--create a temp table like:
select * into #tmp from
(select 1 as ID
union all select 2
union all select 3
union all select 4
) as A

-then run this query:
select B.ID,isNull(B.GID,0) as GID from
(select #tmp.ID,A.GID
from
#tmp
left outer join
(select ID,1 as GID from #tmp where ID in (2,4)) A
on #tmp.ID = A.ID
) as B

--I thougth it should return:
10
21
30
41
--but it actually returned:
IDGID
11
21
31
41
--if I change GID value in '...(select ID,1 as GID from #tmp where ID in (2,4)...', for example 2, then it return:
IDGID
12
22
32
42

WHY?