Why *no* ambig·uous complain in select part?

[Emi Lu <emilu(at)encs(dot)concordia(dot)ca>]

Good morning,

Just notice one small thing, and need your information about select

select col1, col2
from table1
left join table2
using (col1, col2)

;

This query never complain about ambiguous columns of col1 and col2 in 
the select part.

My guess is:
(1) col1, col2 always return table1.col1, table1.col2
(2) because using (col1, col2)
    that's why, table name is not necessary in select part

Am I wrong? Please advise?

Thank you!