Wouldn't that statistic hold if Toronto were building the normal number of skyscrapers, but were particularly slow about it? If a city starts 1 building/year and buildings take 5 years to build, the steady-state average will be 5 simultaneous active construction sites. On the other hand, if it takes 10 years to build a building, there will be 10 simultaneous active sites.
The more likely explanation is that the author has excluded everything less than 100 meters in height. That's somewhere between 30 and 35 floors. You'll be using a "permanent" construction crane for anything greater than 10 or 12 floors - a lot of buildings that use a crane are simply not being counted.
FYI - in high-rise construction, the foundation is designed to have a space for the construction crane. It is anchored to the foundation at the beginning of the project and removed at the end. IE, for the duration of the project the crane is part of the building. Shorter buildings will have a "portable" crane, even if it is on-site for a long period of time.
