I put in the dates you mentioned, June 28 - July 21.

Oasis

YVR - HKG return was $897.31 Canadian (369+339+tax) (GBP442 at the UK site)
HKG - YVR return was $927.31 Canadian (369+369+tax)

Non refundable, but changes without penalty with 24+ hours notice. Note: for $70CDN more each way, a refundable fare was available.

Air Canada was

YVR-HKG $1608.33 Canadian
HKG-YVR $ 845.37 Canadian

Non Refundable $100 and $200 change fees were hard to decypher.

So, you're right with the ex-HKG prices. Looks like AC is cheaper.
But, Ex-Canada the price is much higher.

Fare rules are much more generous with Oasis, but for many this is not a concern.