To explain, tour operators only provisionally book capacity on holiday carriers. If they sell all the seats, that's fine and the arrangements can go ahead. If not then they do not operate, and "consolidate" on other operators. There is a fair amount of commercial discussion around this as you can imagine. It's normally finalised about 6 weeks in advance, 3 weeks is a bit short but probably reflects the time taken for the news to filter down through multiple hands. It's not just low demand either; high demand can mean they sell their own aircraft/seats out and then start shopping around the others.
Broadly speaking the major operators with their integral airlines are more likely to operate as planned, whereas the independent/untied airlines who come and go by the year are more likely to chop and change. It's one of those things where, the further in advance you book, the more likely things are to change. If you go and buy a holiday for next week you can pretty much be certain it will be as stated.
Holiday charter travellers will have noticed that all the different operators tend to go to the same place on the same day, from every departure airport in the UK; if you're say going to Fuerteventura most flights are Wednesday, a few on Saturday, and little on other days. In part this is so they can readily match different combinations of accommodation with different departure points if it all changes over on Wednesday. But it also facilitates fluidity of airline arrangements on the day, and "pushing everyone around".
"Ghost flights" can often be spotted at booking time by :
- Attractive departure times like 09.00 rather than 06.00.
- Departure times at "round" hours.
- Actual airline for the flight not mentioned, just a selection of names shown.