NT + deadlock intended behaviour ?

Hi all,
I'm doing some experiments with NT, I din't expect this behaviuor:

create table test ( a integer );
insert into test values (3);
insert into test values (4);
insert into test values (5);
insert into test values (6);

SESSION 1; SESSION 2;

begin; begin;
update test set a = 300 where a = 3; update test set a = 40 where a = 4;
begin;
update test set a = 400 where a = 4;
<BLOCKED>
update test set a = 30 where a = 3;
<DEAD LOCK DETECTED>
commit;
<UNBLOCKED> <-- !?!?!
<here I'm able to do another commit>

why SESSION 1 was unblocked ? If I repeat again but I do an abort:

SESSION 1; SESSION 2;

begin; begin;
update test set a = 300 where a = 3; update test set a = 40 where a = 4;
begin;
update test set a = 400 where a = 4;
<BLOCKED>
update test set a = 30 where a = 3;
<DEAD LOCK DETECTED>
abort;
<STILL BLOCKED>

Why that commit unblock the SESSION 1?

Regards
Gaetano Mendola