If you are not a status passenger (Tango Plus doesn't give you access to the front Y cabin unless you hold status) and the flight is fairly full, chances are there will be no more seats left for advance seat selection as some seats near the back end of the cabin are held for day of departure. If it's a 319, last 3 rows are held for day of departure (18 seats) + seats held in the front cabin for status pax or those who are willing to pay for exit rows, that's already more than the 30 empty seats that you are quoting.