Welcome to FT, puntamita!

The seat map is not representative of the load on the flight. It is possible to have a reservation without a seat assignment (which you could be seeing in First), and it is also normal for the airline to block some seats until very close to departure (which you could be seeing in Y, where it looked like a lot of seats were filled/blocked, but nobody was really there)

When the reservation agent said that first is sold out--are you still able to go online and book a ticket? If it's available for purchase, then it's not sold out. If it's not available for purchase, then the empty seat on the seat map is irrelevant