It's because we were able to run, during the manufacturing process, 20 colors for the X and the O, which is the name of the computer, and by mixing them on the manufacturing floor, you get 20 times 20: you get 400 different options there.
Nes mums pavyko gamybos proceso metu taikyti 20 spalvų raidėms X ir O, tai kompiuterio pavadinimas, o sumaišius spalvas gamykloje gaunasi 20 kartų po 20, tai 400 variantų.