Twins are simply cheap to construct. Just reuse or mirror the design you already have and boom you don't have to go through the effort to design a new building. Most twins in London are exactly this, with the exception of City Centre and Two London Place (if it gets built) which are shorter.
The view from the market will look even better if Covet Place is built. See some pics of it in the downtown vision:
http://www.london.ca/Planning_and_De...pt_28_2011.pdf