Because in all previous years, HK had always had the US / International model, last year the XS was the US model with e-sim. And in all those cases the prices were $100 more expensive. You are picking the "specially made" in all model this year and one model last year, while the price disparity has been there since iPhone 5.
i.e The price difference didn't happen because there was a special model made for that region, it has been like that for years.
Not to mention Japan has its own model as well due to Camera and Felica NFC for a long time, none of these had any effect on pricing.
iPhone 11 64GB. HK$5999 = $767. Or $699 in the US. Not a $100 difference.
iPhone 11 Pro Max 512GB. HK$12499 = $1598. Only $1449 in the US. A bigger than $100 difference.
The claims of an arbitrary $100 increase are plain wrong. It is % based. And probably necessary because of the cost of logistics and running... 5 is it now?... apple stores in a city half the population of the mainland Chinese city next door which has a total of 1.
The Japanese model is an oddity and no longer exists in the iPhone 11. It was a temporary measure until technology caught up, ie something Apple would suck up the cost difference on if any. The China/HK model has a different background. Japan is also worth mentioning since in the past, the cheapest place in the world to buy an iPhone was at one point Japan, even with sales taxes included and compared to Hong Kong. This was due to currency fluctuations, of course, rather than Apples pricing strategy.