Different materials and thicknesses of those materials can yield different ranges of consistent COR across the face. Likewise, a manufacturer might opt for different material for the benefit of more or less weight in certain areas. Durability could also be a factor, as stronger steel may yield a thinner and lighter face allowing further variation in auxiliary weight around the clubhead.
But if you're swinging well it just doesn't matter that much. Get something relatively modern that you like and play.