Diferences between IN and EXISTS?

Hi!

Assumptions: PostgreSQL 9.2.2, Win64, already run vacumm full freeze
analyze. No problems in the database. I know there are 1247 records to
be found.
Why does these queries return different results:

select count(*) from parcela
where id not in (select parcela_id from cadastroservicoparcela);

count
--------
0

select count(*) from parcela
where not exists (select 1 from cadastroservicoparcela where parcela_id
= parcela.id);

count
--------
1247

I can't figure out, I did expect that the first one returns exactly same
result!
Could null values in cadastroservicoparcela.parcela_id affect the first
query?

Thanks,

Edson