How to recall all top_k docs?

[chansonzhang]

searcher.search_all(k=top_k, ...)

I want to use colbert as recaller and return all top_k docs. However, the recalled docs is less than top_k.
I guess there must be some threshold mechanism, so I tried to set centroid_score_threshold=0.0 in ColBERTConfig, but it still not return the all top_k docs.
How can I achieve this?