Looking at the algorithm for ring signature, n members of the ring can have m keys associated with them.
What is a scenario where the other members, not the signer, would have multiple keys associated with them.
In other words, why would the decoys have multiple keys associated with them?
We assume that if a tx has multiple outputs, then all of those outputs belong to the same person. So when we select the decoys, we take all of those outputs and say it is one person's set of keys.