For some reason reason 4 to 6 months sticks out. I think this is because ticket prices increased last June and August the year before. This made the ticket pricing into the next year known so the packages could be made available.
As far as price increase, 10% to 20%. 10 at the low because of growth revenue the company tries for. 20 at the high end because of decreased discounting.
When you compare it's really easy to be comparing apples to oranges. You have to compare this years verse last years rates equally, and it's difficult. Things like Spring Break vary each year because of how Easter Falls. This moves Spring Break around and effect the demand for a particular day. Christmas week likewise can change too because Chrismas and New Years can fall on weekends or midweek. Even the weather can effect things, some people see rain and cancel to shift a week back.