This is a provincial matter so the rules you are referring to are in Ontario.
If you arrive with no foreign drivers license or a brand new one, you may take the written exam to get the G1 license but you must do your one year waiting period before you can do a road test and get a G2 license. If you take a course at a approved driving school, the waiting period is cut down from 12 to 8 months.
If you have a foreign license and 1-2 years driving experience, if you have a drivers abstract from police or transport authority in your country as well as an English or French translation of your license and drivers abstract, you may challenge the basic road test right away and get a G2 license without a waiting period.
If you have more than 2 years experience, you can challenge the advanced road test and get a G license right away.
You can read about this here: http://www.drivetest.ca/en/license/exchangeoutprov.aspx
I also think these rules are not very friendly to immigrants who do not have a license already. In Canada, most people are taught how to drive by their parents during their teenage years so it does not really matter if they can not drive alone for a year. It is their practice time. For an immigrant however, especially one who is alone in Canada and needs to get a license, since they will not know anybody to drive around with, the practice year becomes a waiting year and it will not at all make them a better driver.