I really do not have any Gemara to look this up, and  it occurs to me  that without a Gemara to look this up I might be wrong. Still I would like to suggest this idea: Abyee in Bava Batra pg 24 derives from a mishna that רוב וקרוב הולכין אחר הרוב what does that mishna say? It says דם הנמצא בפרוזדור ספיקו טמא שהזקתו בא מן המקור the Mishna says there is a חזקה and that חזקה makes the blood to be a doubt. And Abyee derives from this a ''most'' רוב is no doubt. רוב וקרוב הולכין אחר הרוב  Thus what this must mean is from the middle of the hallway and upwards, the blood is a doubt because of the  חזקה/ And from the middle and downwards the blood is definitely impure because of there is a "most". Most of the blood in that area is from the source. This fits in the Gemara in Bava Batra and is what Rav Huna said דם הנמצא בפרוזדור מן הלול ולמעלה ספיקו טמא מן הלול ולמטה ספיקו בודאי טמא that would be because from the middle and down there is a  חזקה ורוב and from the middle and up there is only a  חזקה

This seems clear if there would only be the Gemara in Bava Batra. But Abyee asks on Rav Hunaa in Nida pg 17b."if you go by a doubt then the blood found in entire hallway should be impure. If you go by חזקה then the blood found in the lower area should be considered impure and blood found in the upper area should be considered definitely pure. This sounds more like a question on the Mishna. How is it that the Misshan says because of a חזקה there is a doubt? Rather is we have a חזקה there should be no doubt. And if we have a doubt then there is no חזקה This I think explains why the Rambam brings the law as stated by Rav Huna and ignores the question of Abyee, The reason is that Abyee himself clearly agrees with the explanation of Rav Huna in the Mishna in Bava Batra. For there it is Abyee himself who says from the Mishna we learn.רוב וקרוב הולכין אחר הרוב That must mean that even Abyee thinks חזקה would not be enough to say the blood is definitely impure and we need a "most "also. This is exactly like Rav Huna. So Abyee asked his question in Nida 17 and then later agreed with Rav Huna. 

I should mention here that I do not recall all these gemaras off hand but rather aw them in the book of Rav Shach in Laws of Forbidden Relations 5 law 5. There I see he has a deep explanation of the Rambam. But here I was concentrating just on the statements of Abyee and Rav Huna. Just the logistics, not the deeper reasons for what they are saying. That I leave to Raav Shach.