The island hop does not run every day. I think that it is only available for MWF. So, this reduces your search time.
I found a similar fare for $526 from IAD (can drop last segment to YUL/YYZ). It is not as great as a deal as last year, especially for needing HKG-PEK flight instead of a train ride from HGH to PVG. For experiencing the island hop, it may be worthy. Cheers!