Yea I don't understand the reason either. I can venture a guess though. I've talked to couple of people who applied for a 6 month visa but got a longer term visa (to the length of expiry of their passport). So I would like to think that the VO might want to give a visa longer than 6 month. And medical is required for visas longer than 6 months visa based on CIC website.
That's just my theory. I'm in the process of getting my wife's medical done. We'll see what kinda visa she gets.