Yeeeah, pretty sure that they all deal with different portions of the world, not just the 13th. I mean, altogether the Gotei has about 3600 members divided somewhat evenly. Roughly 277 aren't going to cover the entire planet earth. Especially not when you consider that seated officers don't actually do that kind of grunt work unless there's a hollow deemed enough of a threat to call them in. And if it was just the 13th that handled earth affairs, what exactly would the rest of them even do? Sit on their thumbs all day?