Why IndexIVFFlat search empty invlist?

[Kyungmin-Yu]

I used IndexIVFFlat to search for 11 nearest items, and the result included -1.

Since the dataset size is (60,100) and I used n_list=45 & n_probe=15, I expected to get full result.

Therefore, I invested which cell was searched by this code.

quantizer = faiss.indexFlatL2(100)
index = faiss.IndexIVFFlat(quantizer, 100, 45)
index.train(vectors)
index.add_with_ids(vectors, ids)
index.nprobe = 15

D,I = index.extract_index_ivf(index).quantizer.search(query, 15)
invlists = index.invlists
[invlists.list_size(int(c)) for c in I[0]]

It turns out that index inspected cells that contains 0 items.
I wonder why there is some cells that is empty, and why is IndexIVFFlat selected this cell to search.

Thank you.