Didn't Sun and Moon have extra day of tracking during first week compared to X / Y? I think that would explain bigger drop to second week.
Oh, right, there's that too (completely slipped my mind, to be honest). But there was the public holiday on Wednesday, so that should have helped sales a bit (well, it seems to have helped with 3DS HW, at least).
I guess we will see with Famitsu (and Dengeki) data, as I'm still wondering why MC alone had a better opening for SM (and it was a significant difference with Famitsu / Dengeki).