That is pretty damn low tbh.

Ono Erena's solo debut single opened with 9,989 and that's after a long absence. She got a 25,740 weekly.
Kasai had a solo live, various (upcoming) events, TV show appearances, not to mention she's a semi-popular AKB48 member...
edit:
Wasamin opened with 7,352 at the time, ending up with 23,773 for the weekly.
First week targetsQueen & Elizabeth - Love Wars: 21,179
Iwasa Misaki – Mujin Eki: 23,773
Ono Erena – Erepyon: 25,740
The only other singles that sold in this area are hella old unit and akb singles, which aren't really worth comparing...