Actually, it is the same in Vaughan (Int. 30 vs. Colossus) and in Scarborough (KC20 vs. Coliseum). The two theatres are in the same catchment area so only one can show a particular movie at a time. Distributors will decide which theatre gets to show their movie based on a number of factors such as how screens the movie will be shown on with x amount of seats. Usually the theatre that can bring in the most revenue will show each movie. Some distributors will just automatically have their movies run at the same place because both the distribution company and theatre are owned by the same parent company (i.e. Viacom used to own Paramount and Famous Players, therefore all Paramount movies went there instead of the AMC in the same catchment area).
The AMCs listed above will run movies longer because they have more screens and can show more movies at one time. Some movies will start to be shown at an AMC after the other theatre stops showing it. There are people, myself included, who will wait a month or so after a movie is released to see it at AMC after its run at Cineplex is over.