Longfufu and Khaleese have never seen any posters in my line of business on here before!
hellooo!
Can I just add a few more points?
People in the industry managing the stock are threatened encouraged to sell through by the end of the season. You never realistically fully sell out in season unless something is an absolute best seller.
The remaining terminal stock you clear in the end of season sale. If you haven't liquidated the stock there is pressure from finance for you to mark it down and clear it by a specific date. A company's health is measured on the season age of the stock. If you have a load of out of date winter then a lot of money is tied up that you cannot spend on future trends. You will also not have the space in stores and distribution centers to store all this stock. So you have to make way for your new stock. So you have to put the money where the least risk is I.e. the season ahead.
Although you try and phase it in, you are dealing with congested ports, and blocked factories, Chinese New Year, queuing containers in Bangalore (I am long in the tooth ladies - I've seen it all [pirates stole my velvet, toy trains fell into the docks! ], so the only way to guarantee you get your stock in before Walmart is to book it earlier and earlier.
I work for a global company, but even I had to queue my production in so early that my SS13 flip flops came in at the beginning of November! If I didn't get them in then, they would have arrived by July due to prebooked capacity.
Yes seasons have always been the same, but factory production in the Far East now has to provide product to a lot of emerging markets with new found wealth now gaining speed in retail - the Middle East, India, Russia, China and many eastern Eutopean countries have growing demands in Consumerism.
God I've written an essay, and I'm supposed to be on holiday! 
Incidentally - out of season stock tends to stay on the shop floor longer in Outlet stores 
Our outlets still have winter down jackets and gloves and sales have actually dropped off a cliff, regardless of the cold snap. I can't afford to have 48 items of clothing on a fixture when I am selling less than one a week! My tee shirt sales have risen by 165% on last week, they need the space.