SQL & PL/SQL

Hi,
I have a question about the count(*) with nested query.

I have a table T1 with these columns:
C1 number
C2 number
C3 number
C4 number
C5 number

(The type of each column is not relevant for the example.)

This query:
select C1, C2, C3, C4
from T1
group by C1, C2

it's not correct becausa C3 and C4 are not columns specified in the GROUP BY expression.

If if run this query:
select count(*)
from (select C1, C2, C3, C4
from T1
group by C1, C2)

I haven't an error message (the result is correctly the number of records).

Why?

Thanks.

Best regards,
Luca