This really doesn't come as much of a surprise.  Many posters have already stated a number of reasons, plus the article covers the rest with the exception of China looking at more aggressive measures of stemming capital outflows, apparently.  

Montreal tracks purchaser nationality and has better purchasing data all around; smaller ethnic enclaves relative to Toronto and Vancouver as two large groups (Chinese and Indians) love to deal with their own; and not as many post-secondary students to buy houses for, or under students' names, but with McGill that's a growing number.  Plus they seem to be very proactive rather than reactive regarding curbing speculation by taxation measures.  But Montreal will still grow and be a spot worth watching imo, just not for ridiculous monthly gains.

Calgary is much of the same along with lesser known post-secondary institutions than the big three of UofT, UBC, and McGill.