It’s quite a complex topic, but I try to explain it understandable.
First of all: On which server are you playing? There are quite some important differences, especially with the introduction/test of the new demand distribution system.
You are right, real life demand data is used in the game with some exceptions and modifications. Important to consider here especially for holiday destinations is that charter flights are not considered. There is also no seasonality in the game which is important for major holiday destinations.
The modification of the real life demand is a bit harder to explain. The issue is that there are only demand data between to airport pairs available, not the real start and end airport of a passenger. So if you flight from Canberra to Hobart via Melbourne we don’t see this in the data, we just see that a passenger flew from CBR to MEL and from MEL to HBA.
To consider this the demand distribution is a bit adjusted within regions (political, cultural) - how exactly is a secret. But I try to illustrate it on the CBR to HBA example.
Let’s first look on Hobart. There is a huge demand for flights to MEL and SYD, decent to BNE, but that’s more or less it (beside some infrequent flights to few other airports). MEL on the other hand as major hub has frequent flights to a lot of domestic (and international) airports - also to CBR. The same is valid for SYD.
What now happens is that some of the real life demand between HBA and MEL is partially redistributed to the other destinations, so HBA to MEL and MEL to CBR demand is reduced, but HBA to CBR increased. Idea is to somehow simulate transferring passenger, though of course this approach can never perfectly simulate the reality. Also the entire topic is way more complex, but I hope you get the point.
Generally you can say that smaller airports that have only very few, frequent destinations (major hubs) in real life get because of the redistribution in the game way more destinations but with smaller demand. This should also encourage to create a hub and provide a lot of connections.
International flights like from SYD to CGK it get’s even more challenging. Not only because of the redistribution of demand, also because of increased competition on longer flights and how demand is distributed on the flights.
Beside the new game worlds Domination and Smiths, demand is distributed depending on the frequency times the rating with only a small penalty for transfer. So as longer the flight is as more competition you might have cause of of competition from other hubs with transfer flights. So if you compete with thousands of other flights, though these are transfer flights with lower rating, the sheer amount of flights makes it quite hard to fill the aircraft (so a working hub with connecting flights is a must)
Hope this helps you a bit to understand the challenges