From airline pilot forum...Delta hasn't signed an official commitment for any CSeries yet, they have just leaked to the press the 75+50 info as well as the ~37 A321s they plan on ordering. My bet it they are giving Boeing and Embraer one last chance of lowball offers before the April 28-29 investor day. This is far from a certainty (the CSeries).
https://www.airlinepilotforums.com/delta/94251-dal-about-announce-c-series-order-19.html