If the answer to a question is not inside the context provided

[tqye2000]

I found something strange which I don't understand how it works.
With the following code

search_index = FAISS.from_documents(source_chunks, embeddings)
docs = search_index.similarity_search(query, k=3)
results = chain({"input_documents": docs, "question": query}, return_only_outputs=True)

I will get an answer from other sources like Wikipedia, if the answer to my question is not in the docs provided.

However, if I use the following code to use k=1, i.e. just return one doc,
docs = search_index.similarity_search(query, k=1)

then I will not get an answer from the "outside" sources, if the answer to my query is in the docs provided. It only says something like "I don't know the answer".

The prompt is also the same for both cases. Why are the outputs different?