Their methodology appears to be garbage.
You have it backwards. The chart says $0.17 postpaid, $0.07 prepaid.
The chart implies a cost of 120 USD per GB, which implies it's based on a 500MB/month plan costing 60 USD. There's no way in hell the cheapest way to get 500MB in Canada costs 60 USD. I'm in Canada and I pay 40 USD for 8GB per month.
The only way I figure they can get to such an absurd number is that no one sells 500MB plans in Canada anymore, so they had to choose a zero data plan + 500MB of additional data to make up a "500MB" plan. With my carrier, additional data is 104 USD per GB.