Why is the lower deck of the Airbus A380 curved like a single deck airplane? I recently traveled on the lower deck of an A380 and the section where the side walls of the fuselage and its roof top meet is curved just like any single deck airplane.


Credits: Wikimedia Commons

If there is a deck above me (which there definitely is) then why does the lower deck have to be curved near the top? Shouldn't it be straight on the sides?

Shouldn't it be straight when there is one more deck above it? 
I'm sure that it's just the interior that is curved and the external structure is indeed straight but does it serve any important purpose?

Details:

Airline:  Emirates
Aircraft: A380-800
Seat: Row 58

Here is what i saw:



Notice that even the roof is curved, is there a structural reason for that as well? On the side wall one of the rows has an extra covering on it, I'm almost sure that is to cover one of the brackets as mentioned in the answer below but even without that the whole cabin is curved.
The cabin interior is curved to accommodate, among other things, the brackets holding the upper deck floor beams.

These brackets are a structural feature that transfers the weight of the upper deck to the fuselage with considerably less reinforcement that a straight T-joint. The bracket acts as a small truss element, creating a triangle with the deck and the fuselage that partially turns bending loads into axial ones.

Here is an actual production cutaway, noticeably different from wiki's simplified representation of the A380's cross-section:



It would be possible to only wrap the interior lining around the brackets themselves, not along the whole length. But they're not the only thing there (although others could be rerouted; structure requires testing), and this curve saves a bit of weight, so Airbus curves the whole interior by default. An airline could request something different if they wanted to.