Oh man, so you're going to
absolutely love this.
Malaysia, Singapore, and certain countries in the Middle East... do fall under the USA region for the 3DS.
The 3DS firmware for that n3DS is USA.
The actual countries where the n3DS was sold in (or to be sold in) was Malaysia/Singapore.
Malaysia, Singapore, and other south Asian countries get lumped together as part as the Middle East market territory.
Super confusing, right?
What might have happened is your n3DS was part of excess inventories that never made their way to Malaysia and Singapore. These were exported in bulk to another country under the USA region umbrella.
This happens a lot for N3DSXL special editions like these (read the reviews):
The download codes probably only work in the intended countries that 3DS was supposed to be sold in. For example, the code is only good in Singapore or Malaysia, but the user's country in System Settings isn't one of those.