The European FB hotline is open today. I just rang them, and their explanation for the odd 210K miles was that this was probably a combination booking: outbound skyteam redemption, return open. Although, the agent on the phone seemed a bit bothered by my call and did not actually check. She was speculating.
I did not realize that a combination was also possible.
When I asked her about ensuring that that remaining 3 tickets would also be booked on the same flights, she said that for open miles were was full availability.
Not sure what to do now ....