Long ago, I had read (PVR? I could be misquoting him...) that the tanu-dhan-sahaj-sukha-suta etc house attributes are reflected in the amshas, so D1 for physical matters, D2 (hora) for wealth etc, D3 for siblings and strength related, D4 for contentment. It is not perfect, of course, because D7 and not D5 is prescribed for progeny... etc and then beyond D12, the cycle of 12 begins again at a higher spiritual plane etc. If following this line of thought, since 9th house is about bhagya (luck, fortune etc), D9 represents the bhagya domain of the 12 houses in D1.
KAS system has another explanation which presumably deals with-explains the distribution, differently.
Of course then there are folks who insist that the amshas must not be treated as bhavas and so while navamsha might indicate a measure of strength and beneficience of planets in D1, it (and other amshas) must not be treated as *charts* with bhavas etc.
As a slight aside, we also hear from modern doyens that griha drishtis (planetary aspects, such as conjunction, sextile-square (saturn), trinal (jupiter), nadir square (4) and quincunx (8th) in case of mars) must not be mixed up with the rashi drishti (sign aspects, such as fire-water etc) in an analysis. This I cannot instinctively agree with since planets do not wear either Parasara hat at one instance or Jaimini hat at another! Just does not feel right to me. Moreover both types of aspectual relations are described by Parasara in his scriptural text! Speaking of which, there seems to be a bit of discrepancy because in readily available sanskrit-english, sanskrit-hindi versions, chapters and slokas are differently presented. For instance the sloka quoted by you in a few of your recent messages (chap 3 sloka 5) shows up in the Chowkhamba edition in chapter 3 but as sloka 8 and more importantly is differently worded! Modern problem arises as to which version to believe in (preferences vary all over the globe) and I have also heard unsubstantiated claims that there are half a dozen versions floating around, some revealed, others partly or entirely kept hidden by possesers of those for a variety of reasons! Some even opine that the original full BPHS is either lost or *rephrased* and/or patched up by folks later on, and not necessarily by nefarious intent! I have captured some of these in my article about "lost astrotreasures over time...".
It is understandable and indeed expected that given the huge number of years between the original discourse between Maitraya and Parasara, much water (of all kinds!) has flown under the bridge...!
Munde munde mati bhinna, I suppose...? As many birds, so many calls...?? Sure emough to keep interested minds occupied for more than one lifetime!