How can two species that look very different from each other be more closely related than two other species that look similar to each other?

Some animals can be of the same sub-order, and be therefore related because they share certain characteristics, even though they look different. An example is the fact that koalas and wombats are both marsupials of the order Diprotodontia. The koala's family, Phascolarctidae, is close to the wombat family, Vombatidae because they are both of the sub-order Vombatiformes. Yet koalas and wombats are very different in appearance.

On the other hand, pandas and red pandas may be of the same order (Carnivora), but they are of different sub-orders.