I believe Times group is hoping Buttonville will be closed by the time the building currently under construction actually reaches 20 floors or so. It is supposed to close in the next 12 months or so, which will be before the building reaches that height.
The limit is around 20 floors in Downtown Markham from what I remember, thus all the relatively short buildings that are all shortly under that number.
You can see in the graphic that they are only doing the rezoning for those extra floors now, despite having started construction. They are cutting it by the skin of their teeth, for sure.