So there are two parts to this… For the first part (Default netclass being present) - this will always be the case if you have some elements of the required netclass undefined. As there can be multiple netclasses assigned to a net, the resulting netclass takes a given field’s value from the highest priority netclass on which it is defined. These values are always defined on the Default netclass, so it’s being used to fill in the gaps. Note that the netclass settings table has some hidden columns, depending on whether you open it from the schematic or pcb editor. You can always right-click on the column headings to enable the others. This isn’t great UI, and we are thinking of ways to improve it.
For your second point, the condition "A.NetClass == 'IBUS' matches the full netclass name of object A (likewise for object B in your example). As you have the Default netclass included implicitly, the full name is actually IBUS,Default. Much better in v9 is to use the condition A.hasNetclass('IBUS'), as this will test all netclasses assigned to an object.