Firstly, it's crowdsourced from this spreadsheet http://nomadlist.io/edit/ so it might not be 100% accurate.
Secondly, NomadCost != cost of living. NomadCost is based on short-term staying in a hostel, hotel or apartment in the center, working in a coworking space and having a basic meal three times a day. That's the average digital nomad's lifestyle. They move around every few months, so they can't rent long-term. So NomadCost will be way more expensive than cost of living for a resident.
I'd like to monetize this by selling city specific nomad guides on how to set up in each place and letting people find jobs remotely. Hope this helps! I think this is the future of work, so I'm very happy to help push this.
P.S. this is part of my goal to launch 12 startups in 12 months (see http://levels.io/12-startups-12-months)