The only other plausible explanation I can think of, is that some Tor exit nodes are running on high-end machines without their owners' consent or knowledge.
Because no one owning, working or operating a ISP would ever, ever, dream of sponsoring some cable time to an anonymity network. They might be technical people who's job it is to work with computer networks, by why would they be interested in computer network technology? That would be like a technology company sponsoring open source project with work hours.