score:6
It just means that there are expensive departure dates because they're popular. 3 Dec to 15 Jan is the Christmas rush period as school ends and so on, while the June one is the other big holiday period.
It'll be based on your first flight, especially as out of Australia where you are, that's going to be one of the biggest.
(Just looked at the page, that's a pretty good rate for a RTW ticket!)