The reason you are supposed to know is that they tell you (vaguely) in the T&Cs - and you quite literally tick a box to say you've read them. In fact under English law that clause may not be valid depending on why they alter the times and by how much but it rarely gets challenged.

Actually the times may not be based on any previous years timetable, they could be just using some provisional "back of an envelope" plan especially in the charter market. A couple of years ago MyTravel were selling seats on a flight out of Faro at 00:50 before they had it confirmed. In fact they had no chance of getting it confirmed because their so called planners hadn't noticed that Faro airport shuts at 23:00! When they eventually realised they simply altered it to go at 20:00, which meant the outward flight from Manchester ran over 4 hours earlier, which meant people who had thought they could get to the airport after work/school had to take the afternoon off. Buried in the T&Cs was a clause that said they could vary by 12 hours which they used to stop most people claiming their money back - with one exception..........