Not sure of the specific reasoning in this case, but the short answer in the abstract is that it depends -- different suppliers, different floorplates, different floor heights, different programs, different engineers, different architects, different builders, different pro formas; those are all factors that generally factor into the decision, and it tends to be iterative to at least some degree.
In broad strokes, it has for some years been true that, ceteris paribus, it costs more to do your core and/or one or more of the first floors in timber, but there are definitely suppliers, engineers, and builders out there who are claiming that the cost gap (for at least the latter) has substantially narrowed. Even though there are now lots of tall timber buildings all over the world that are pushing the envelope to a greater degree than which we're seeing in Toronto, there's still a pretty widespread concern here among builders about the bench strength of trades who can work with it here (especially given that there are now actually quite a number of mass timber buildings under construction/in the pipeline).