There are a couple different reasons that can happen on Geni.
Geni only knows about the relationships that it has been told. If the path hasn't been created on Geni, then Geni doesn't know about it.
Another thing that can happen is if you are talking about DNA testing of people of Jewish heritage. 9th to 11th cousins could appear to be like 2nd/3rd cousins due to the amount of shared DNA.