I heard the length of validity of visa has something to do with reason for visit. if you have a very specific reason to visit, like attending an event that only happens one time in Canada which will relocate to other country next time, they will issue visa just to cover that period of time. anyhow, if your visa covers the whole planed trip, every one is happy. in addition, visa can't not be issued longer than the expiry date of your passport, biometric or 10 years, whichever comes first.