Because the fuselage is narrower on the upper deck so you can't get more than 8 across, where as on the lower deck the fuselage is much wider, thus accommodating 10 abreast.

Your conundrum is a familiar one. Grab one of the better seats downstairs but risk having some random sat next to one of you, or sit as a pair upstairs in a more private, quiet cabin but without the extra legroom of the seats that have been mentioned (which only one of you would benefit from anyway). If I was alone I'd go downstairs, but with a partner I'd go upstairs.

One consideration for such a long flight in favour of taking the bulkhead is that you avoid the scenario in the first photo in this article. I'd say save your knees and when you want to really stretch your legs you can get up for a walk!

http://www.smh.com.au/travel/blogs/t...203-2aq0p.html