Airline ticketing is super complicated. First of all, which bucket a given leg books into is defined by the fare basis you're attempting to book, from origin to destination, not on a leg by leg basis. So just because a lower bucket exists for one of your legs, which one you book into depends on which fare you've been sold, and you can't substitute a lower fare for a portion. And not all fare buckets have a published fare basis for that origin and destination city pair at any given time (or ever).

Even though there are lower fare buckets than H on the TPAC and U on the domestic, there are no fares published between YYC and HKG that will book into them. AC may well publish such fares in the future, but they may equally well not. Depends on booking patterns. At this point, since H is so high, you may be better off waiting, but there's never any guarantees.

Different airlines have different strategies for managing their inventory, and it varies widely by route. United is likely being far more liberal in releasing lower fares because they don't feel they have a "lock" on the YYC-HKG market. If you searched something like SEA-HKG with AC, you'd likely get a similar result.