I also think the problem with an early announcement still stands in pushing fans of that artist to make it a ticket sale for them, which adds to the numbers and increases the chances of decent numbers buying tickets with the intention of coming in for just one day.
Both points make sense, for these sized festival getting a reasonable headliner makes all the difference to its appeal and thats where a big chunk of the budget goes.
Still looks a little odd not to have put one of the headliners on the first poster release but get that they would hope what they released would get some more money in.