Jim Ley wrote:
So why have a range at all, if these flights sell out immediately? it
seems rather strange, anything that sells out immediately normally
suggests you can charge a higher price, especially if it's selling out
immediately 11 months ahead!

And just because there won't be a seat sale on those dates doesn't mean
that the flight won't have advance booking standard return fares that
are much cheaper than full fare Y class tickets. (aka: what used to be
called APEX fares, standard return tickets booked 14 to 21 days in advance).

When they populate a flight that far ahead, they offer standard/default
booking classes and fares. And even on domestic routes because you often
have to combine a domestic fare to an intl one and often this requires
both be booked in the same booking class.

So, lets say they populate the christmas season for LAX-SYD, they may
offer some 50 seats in V class (standard advance purchase return fare).
Once those 50 seats are gone, you're left with more expensive fares.
Since this is a busy season, you can't expect seat sales to appear later
on, so you want to snatch enough V class seats to fit your family on
that flight before they are all gone. (note: airlines may use different
booking class for standard apex fares, but V is fairly common).

However, airlines don't have to announce a seat sale to "lower prices".
Say the 50 V class tickets sell out 11 months before the flight; 2
months before the flight, the airline may have sold only those 50 seats
and none other, so they may reallocate some Y seats into V seats to
allow more people to book in V. But with experience, an airline will do
initial allocations based on past demand and may not have to
dramatically adjust capacity for each booking class in a major way for
such a busy season.