They collected details of 60 plans[1], which is their max. The cheapest they found was $2.5/GB, the most expensive being $140/GB (both of these are incredible). They use the median of those sampled[2], but there's no weight given to the plans.. the terrible data plan (100MB for $14) has the same weight as a popular plan (2GB for $40).
There seems to be no concession for other features (roaming, minutes, long distance, voicemail, caller ID, text) nor device subsidies (many operators still include $0-200 flagship devices on Canadian plans) which doesn't exactly level the playing field worldwide (every country has different strategies).
So grain of salt required - I pay half their "average". Is North America expensive? Yes! Do I want 11GB/$? Also yes! (Although India's low price could be a side effect of the Reliance Industries investment fever[3])
[1] https://s3-eu-west-1.amazonaws.com/assets.cable.co.uk/mobile... [2] https://s3-eu-west-1.amazonaws.com/assets.cable.co.uk/mobile... [3] https://en.wikipedia.org/wiki/Reliance_Industries#Major_subs...