All this is 100% spot on, and has been happening since the 1980's. Often the smaller operators will go to a flight broker who will offer them seats at a certain time throughout the season. This might be:
01 June: 20 Seats
07 June: 30 Seats
01 Sept: 20 Seats etc... etc...
Sometimes the operators will oversell these and other times have capacity and place them back into the market place for other operators to take them. If you read the small print with a tour operato they never guarantee the flight timings as they are charters, they guarantee you will fly on that day but not the timing, so they try and move things. It is ALWAYS a last resort I know thsi first hand, but in a way its naughty but airlines change regular with timings and slots and this is also connected to airports as well. They may be offered a flight time and aircraft then the airport changes its slot as its a charter and the airline may then need to change aircraft or can not supply it so the broker goes to another company.
Its just business, but life. The ONLY way to avoid it is to book with a company using a scheduled carrier, some do offer this but not a lot and its often higher in price, but schedule is that schedule, charter just hops into the available air space!
Hope that helps.