Originally Posted by
The Lev
If you were a member of Mileage Plus (UA's program), you would have earned 100% miles on both flights, but because AC doesn't want people to fly other airlines for 100% miles rather than AC Tango for 50%, they convinced UA to only offer people 50% when crediting to Aeroplan.
Actually, if credited to UA the OP would have earned redeemable miles based on the amount spent on the tickets - which on long haul flights such as that may have ended up worse than the 50% earned on AP.
Of course, towards UA status the OP would have indeed earned 100%.