Because the air ticket you bought is from some travel agency, that means it is cheaper and so if you change the date, there are some charges. Even you bought directly from Air Canada, if you want to buy the ticket that can be changed free of charge, it will be much more expensive.
the problem is it's difficult to say what is "original price", it changes with season and other reasons, only thing you need to do is ask each time when you buy the ticket if it can be changed free of charge, usually if you want this option, they will charge you $50-$100 more.