Because the second level isn't really involved, they are a formality. This isn't unusual, it happens when the range of things a group does has grown so large that it needs to be primarily managed by dedicated subgroups but the rules don't update to delegate all authority to that subgroup.

The members of the broader group that cared strongly about C++ would have just been involved in the WG21 subgroup in the first place, the subgroup that approved it with a 2/3rds majority. So it almost certainly means that a broader group of ISO that normally don't care about what C++ does (and hence aren't involved in WG21) were influenced to vote in the direction the minority of the WG21 wanted.