The rankings are pretty generic and average. It's possible to find pockets in Brampton that's far nicer to live than, say, the worst part of Toronto. Or vice-versa. I say this, even as a Hamiltonian
Broken down block by block, there's really a massive overlap, with nice parts and bad parts of all cities. And it breaks down even further depending on your goals (nightlife? parks? neighbourliness? city centre? suburbs? etc.)
An example of priorities. Our neighbour gave us an apple pie when we moved to our house in Hamilton. In the neighbourliness department, we found our block in Hamilton unexpectedly ranks many, many, many rungs above many places in 416 Toronto, and even more neighbourly than Riverdale/Cabbagetown (the area we both used to live in); we've made more friends in less than six months, than all of 3 years in Toronto living in two different locations in Toronto. There are great people and neighborhoods, but they can be hard to find. It gets cheaper in places like Pickering and Brampton, and perhaps, the neighbourliness factor goes up (that I don't know -- perhaps you Pickering and Brampton residents can chime in how amazing your neighbours are), which isn't necessarily accounted for in the "best city to live in the world" surveys.
Now... on the other hand, I certainly concede we rank fairly low in public transit (at least until the LRT and major bus upgrades). Easily lower than even transit-inadequate Brampton, and we definitely rank far lower than Toronto for public transit (at least on the major TTC routes).
Also, I have long ago learned the "best city to live in the world" behaves very, very differently with people tight on money, versus people who have lots of money. The ones in the rich international magazines are somewhat biased towards people who can afford any of the city listed in the list. 416 Toronto is a fantastic city to live if you can afford it and have the time to enjoy all the expensive amenities all the time, without periods of struggling between periods of enjoying the city. I suspect a lot more-struggling central Torontoians are so focussed working hard and paying their lifestyle, or super-expensive rent or mortgage, in an expensive city that they have less time for friends, or being able to afford near-downtown Toronto's expensive life (on average, anyway).