It is a computer game that is adhereing to the D&D rules as closely as possible.
It's also supposedly the sequel to an established franchise, in which a party of 6 is a core element. Both "4" and "6" are in your "rules", and there's no rule that says "4" is closer to the rules than "6", so your "adhering to rules" argument is completely meaningless. Not to mention Larian has the ability to change the rules if necessary.
