Sometimes low cost can result in different things. CNRL's spaces are old, and they might want to move to unified standardized plan, and it might make sense to instead move entirely to a new to them space to achieve minimum of disruption. They may view their current space as inadequate quality.
There can be other things informing a large move as well. If a company was moving to a greater proportion of hot desks, and was targeting less than 200 sqft for employee, all around a concept of collaboration, they could try to adopt large floor plate along with it as the style of the time. If the company didn't think that could work spread over 30 different floors, but it could spread over 6 including 3 full donuts, then you have the podium full.
Not saying it is CNRL, just that it isn't always about having the lowest cost per square foot.