The simple answer is that journalists aren't lawyers. Prorogation is a standard part of the legislative process: it occurs not only every time there's an election but every time the Government of the day decides that it needs to set a new agenda. Every time there's a Speech from the Throne, you can assume that it follows a prorogation.

Prior to 1984, the "annual volumes" of the Statutes of Canada were published only at the end of a session of Parliament, so you can see the history and length of the sessions by just reading the spines of the volumes. (Prior to the publication in the volumes, you have to go to the Canada Gazette).