The Northern Water Tribe appear European while most of the Southern Water Tribe look Inuit- except for Sokka, Katara and Kanna (their grandmother). In the show, Kanna ran away from the Northern tribe, so perhaps their different ethnicities are meant to reflect that.
That is the official reasoning; they even made an effort to cast mostly Europeans as members of the Norther Water Tribe. It might even confirm a persisting fan theory that Kanna was pregnant with Pakku's son when she ran away, making Sokka and Katara his biological grandchildren. Still...you'd think that the two kids would look just a little more Inuit, considering that Hakoda, at least, married a Southern Water Tribe woman.