I think this site calculates the cost of living as a transient; staying in short-term accommodation, eating meals out, etc. The spread sheet specifically asks for the costs of meals and hotel rooms, which is probably why it skews high.

I believe this is amount for lifestyle above the average. Discount Macbook Pro for 4 years and other stuff. Same apply for London. You can live off 1000 a month, but 2500 gets you comparable life.

This surprised me as well. "costs of living" on this page are higher than average monthly salaries in these towns.