What is the percentage of getting "Soft Humanity", and what affects it?

In Dark Souls, when you first start off you have a fairly high chance of getting Soft Humanity[1] whenever you kill an enemy. As a fairly experienced player, I rarely die before I ring the first Bell of Awakening, so by the time I have reached the Gargoyles, I already have enough Humanity to Revert to Human, and Kindle the Bonfire.
However, as the game progresses, regardless of how well I might be doing, collecting the Liquid Humanity from fallen enemies becomes less and less of an occurrence.
What affects this?
[1]Soft Humanity: the Humanity that goes straight to your counter when you collect souls after killing an enemy; characterised by a black stream
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Dark Souls Dissected #8 - Humanity Drop
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