How are the types of Hidden Power decided?
Bulbapedia says:
The actual type of Hidden Power is determined by the Pokémon's individual values, and through calculation, can be set as one of other natural types.
But then wouldn't the same pokemon with different IVs have different types of Hidden power, or is it how the game works? If not, how does it work?
Yes, that is precisely how it works.
From the very website you mentioned, we see that, from generation II onwards, the formula to calculate Hidden Power's type is
where a, b, c, d, e, and f are respectively the mod 2 operation of the HP, Attack, Defense, Speed, Special Attack, and Special Defense IVs.
Depending on HPtype, Hidden Power is then assigned a type, like so:
You might notice there is no Normal- or Fairy-typed Hidden Power.
This table gives you the highest possible IVs that give a specific type.
Due to the mod 2 operator, 31 can be replaced with any odd value, 30 with any even value.
