During Week 20, several games came out in the United Kingdom, and one was on Nintendo 3DS: Fire Emblem Fates, which is available in two versions (Birthright and Conquest). Unfortunately, GFK Chart Track doesn’t provide sales data, so it’s impossible to know how the two versions combined would have charted. All we know is that Fire Emblem Fates: Birthright was the most popular version, and landed at #6.
On the other hand, Fire Emblem Fates: Conquest landed at #8. But as mentioned above, GFK Chart Track doesn’t provide sales data (publicly), so it’s impossible to say how many units each version sold. As to why Fire Emblem Fates: Birthright was the most popular, it may be due to it being the most accessible version, and the closest to Fire Emblem Awakening structure-wise.
Since the two versions of Fire Emblem Fates are the only Nintendo 3DS games in the Top 40, they are naturally the best-sellers for that platfom. On Wii U, the best-seller is still Splatoon, but it didn’t sell enough to make it into the Top 40.
Source: GFK Chart-track