Can someone try to explain S.O.S. battles to me? Looking at the route listings on Serebii is a bit challenging at the moment.

I mean, look at pic related. The list is huge and seems fairly nonsensical to me with the Pokémon listed multiple times and in different orders.

Why is Yungoos listed, the Wingull, then Yungoos again? Why is Slowpoke on there four times? Is there a reason that the S.O.S. calls sometimes fail repeatedly with no answers?