Oldest pricing question in the world, and the answer is that the airlines price point-A-to-point-B tickets on the basis of demand for travel from A to B, even when there's a plane change between A and B. The intermediate station doesn't count and might as well not exist. Air travel is not sold by the mile but by the city pair. Many people think longer trips should be automatically more expensive than short ones but this is wrong; a CLE-DTW ticket can be as pricey as JFK-SFO. In your case more people want to go YUL-AMS than YUL-BRU, so YUL-BRU is cheaper.

That would make his costs far higher because two one-way international tickets are typically much more expensive than one discount roundtrip.