There are two potential problems:

1) AC and CP res systems cannot talk to each other. CP agents only see their inventory of the flight, but they have no idea how much AC have sold, and vice versa. So when you talk to AC, they know that they have sold everything that they hold on the CP-operated flight, but CP may have scads of available space which the AC overflow can fill.

2) Flights may show as heavily oversold, and then go half-empty when connecting flights fail to arrive on time. This happens most often on flights between major centres that pick up lots of feeder traffic. This might certainly be the case for UA traffic connecting to YYZ in MIA. It seems less likely in YWG, but anything is possible.