The way I read it:

The OP wanted to have multiple seats on the flight but most probably there was only one seat in the lower fare bucket available. Since the booking engine will not let you book one pax in a different booking class then the rest of the group it will put all of them in the higher fare bucket leaving the single open seat available for someone else booking it later.

At least that's what I have seen at sas.se often enough and made me book us on different PNRs (one cheap, one more expensive) due to that. Of course I think they should just show the lowest fare available for all pax, it seems that is not always the case.