Back in April 2013 Plaza's marketing shows their building renderings as 21 storeys, and their PH floor plans as being on the 24th floor -- they skip 13 and the levels with 4's, so it works out to be 21 storeys. See page 14 of this thread.
I don't know if that was ever changed in thier marketing materials, but I remembering hearing that the height was shortened to 17 storeys, the same number of floors as Oneeleven next door and that was at least a year or a year-and-a-half ago.
Oneeleven Bathurst started quite a while ago, with both Oneelevel and Musee sharing in a common excavation. The Musee side of the excavation activities actually only started in March 2014, beginning with the shoring, which only had to be done on three sides - the west side of the Musee excavation being the common side with Oneeleven. 10 York also started its shoring and excavation activities in mid-December 2015 - it would be safe to say that, in the grand scheme of things, 10 York started slightly before Musee.
Obviously site conditions are hugely different between the two projects, as is the depth of the excavation (which also increases the amount of time required for both the shoring as well as the excavation).