Really then why bother with that model I've always feels weird Captain is the only squad in the game that has 3 types of models.
Not really sure. Maybe they thought it'd make sense for the higher-ranked squad to be using more rear echelon men in their retinue because higher-rankers are farther from the frontlines, so they made it that LT's all Riflemen, Cap's half-and-half, and Major's all REs from that logic.