You've got it backwards - for a change to the "outbound" leg, the fare will be recalculated using the fares that are current at the time of the change. For a change to the "inbound" (return) leg, the fares at the time of the original booking will be used. It is not clear from what you've pasted what would happen if you were to change both legs, however I suspect it would be based on current fares for both legs.
It's a little unusual to see the fare rules differentiate the two legs like this, but I can guess why they have done it - however to explain you'll need to know how most airlines price changes like this.
Generally, which fares are used for a change will depend on whether you are making the change "before departure" (ie, before the first flight on the ticket is used), or "after departure" (ie, after you've flown at least one flight).
If the change is made before departure, it's generally basically treated as if you had cancelled the ticket and made a new booking - which means that the price you would pay for the new flights is whatever they would cost "today" (ie, the day you make the change).
There's a few reason for this, but the main one is that it stops people making purely speculative bookings in order to lock in pricing. eg, I could book a ticket now for travel next June, locking in current "sale" pricing, and then in a few months I could decide I'm ready to fly, change the flights to a few days after, and still get the cheaper pricing. However because this is before departure, the airline would re-price the trip based on the date I made the change, defeating my ploy to lock in the cheaper pricing.
If the change is made after departure - in which case obviously you could only change the return trip as you've already flown the outbound(*), you're generally given the benefit of the pricing that existed when you originally made the booking. With most airlines, you would need to wait until after you had flown the outbound flight in order to make the change on the return flight to get this special treatment (otherwise the change would be considered "before departure" even if it was only for the return leg) - however it appears that Air Canada is giving you the ability to do this even before departure, as long as only the return leg is being changed. (* Ignoring more complex routings for simplicity)
Keep in mind that this only refers to the "fares" that are available at the time. "Fares" in this context is a special airline term that is only one of a few things that controls the price you pay for your ticket. There are a limited number of seats available on each flight at a specific "fare", and if there are no seats on the flight you're changing to in the same fare class as your old flight, then you will need to move to a higher fare class which will cause the price to go up. (This answer will give you a bit of an understand of how these "fares" work, although in practice it's far more complex than that describes!)