ok, I played every gow game out there and if I recall correctly,in every game poseidon is different ? really like his look from the gow 3,but I see in multiplayer when choosing a god he's out for new look again?
Poseidon was the same in GOW 2 and 3. Can't remember if he appeared in the PSP games.
The technical answer is that the game changed hands from GOW to GOW2, and numerous things were altered including appearances.
You can make sense of it still by saying that the Gods are not only vain but also capable of changing their outward appearance at will.
Then there are statues and other idols, which tend to have varied forms.