Suppose a transmitter genuinely heard the same account from two teachers. Both routes could be accurate; their difference alone does not invalidate the report.
In Ziyād’s case, repeated changes of source raised a question requiring verification. His later admission that he had neither heard from nor met Anas supplies direct evidence against a particular claim of connection. This is stronger and different in kind from an unusual but merely possible alternative chain.
A critic therefore distinguishes circumstantial concern, contrary testimony and first-person admission.