Why has Hop let me overbook?

In this scenario, it is important to remember that that if this was a manually entered booking, Hop will allow certain users to overbook providing they have the correct permissions. You can check this by going to Users > Manage Users and looking to see if the person that created the overbooking has the "Can Overbook" activity on their user account:

This is the only way that an individual can overbook a room type when creating a manual reservation.