OK here is the "explanation" (don't shoot the messenger):

1. the fuel surcharge for the reward flight is higher than for the revenue flight because AC did not "eliminate" the fuel surcharge for North American flights - they "included" it in the base fare. This means that on a revenue ticket there is no fuel surcharge for the North American components of your itinerary. As soon as the booking is made through Aeroplan, the fuel charge is no longer "included", and it gets tacked on to the cost of your "free" ticket.

2. Possible explanation for the difference: AC has been reducing the fuel surcharge over the last week or so on various routes. It is possible that those reductions have not yet been transmitted from AC to AE and/or you compared "revenue" tickets after the surcharge drop and the AE ticket prior.

3. When you add a US segment to an itinerary, the fuel charge is lower because AC thinks it is competing for your business against US carriers rather than its more captive Canadian audience, so they sharpen their pencil. For the longest time there was a glitch in their system where there was NO fuel surcharge if you added a US final destination. Sadly that IT issue was resolved.