Welcome to the world of struggling with the on-line tool

I too have experienced your problem. In the middle of a 16-segment itinerary I wanted …BNA-JFK-LHR... The problem was the tool kept choosing BNA-LGA thus inserting an LGA-JFK surface segment and pushing me over the 16-segment limit. (I reached the conclusion that the tool is auto-selecting the shortest flight for the segment)
My solution was to book the itinerary on the phone with AA 😊

BTW – since you are new to RTW planning, have you read the wiki of the Oneworld Explorer User Guide thread? It might give you some tips (but I notice you found on your own that starting in Canada is less expensive than starting in the US )

Happy planning, and in due course happy travels