This is from Bhikkhu Bodhi, The Great Discourse on Causation and explains why there are several ways of presenting this most difficult teaching.
Introduction
p.7 The “depth of teaching” (desanagambhiratii) refers to the diversity in methods used by the Buddha to expound dependent arising. The diversity is dictated by two considerations: first, by the complexity of the subject itself, which only reveals its multiple facets when illumi nated from various angles; and second, by the persons being taught, who can only understand the teaching when its presentation is adapted to their needs and capacities. As the Buddha excels in both “eloquent exposition” and “skillful means,” the result is the great variety in his methods of teaching the doctrine.
From the Commentary and tika translated by Bodhi
CY. (3) Its depth of teaching should be understood thus. In some places in the suttas dependent arising is taught in forward order, in some in reverse order, in some in both forward and reverse order, in some beginning from the middle in forward order, or in reverse order, or in both. In some places three connections and four groups are taught, in some two connections and three groups, in some one connection and two groups.
SUB. CY. Here “forward order” (anuloma) means: through the arising of the condition, the conditionally arisen phenomenon arises. “Reverse order” (paṭiloma) means: through the cessation of the condition, the conditionally arisen phenomenon ceases. But forward order also means starting from the beginning and going through to the end; reverse order, starting from the end and going back to the beginning.
In a teaching that proceeds in forward order from the beginning or in reverse order from the end, there are three connections and four groups. In a teaching that starts from the middle and goes back in reverse order—such as the passage, “Bhikkhus, what is the source for these four kinds of nutriment?” etc. (S.XII,11; ii,11)—there are two connections and three groups. So too for a teaching that starts from the middle and goes forward in direct order, such as the passage: “In dependence upon the eye and visible forms there arises eye-consciousness. The conjunction of the three is contact. With contact as condition there is feeling,” etc. (S.XII,43; ii,72).
In the passage: “When one lives contemplating satisfaction in phenomena which stimulate the fetters, craving increases. With craving as condition there is clinging,” etc. (S.XII,53; ii,86), there is one connection and two groups.
Dependent arising is even taught by way of a single factor. Such is found in the passage: “The instructed noble disciple attends to this carefully and methodically in terms of dependent arising: ‘When there is this, that comes to be; with the arising of this, that arises. When this is absent, that does not come to be; with the ceasing of this, that ceases.’ In dependence upon a contact to be felt as pleasant there arises a pleasant feeling,” (S.XII,62; ii,96). That should be understood by way of this sutta.
Thus, because it must be presented in different ways for different reasons, dependent arising is deep in teaching. Thus the commentator says: “This is its depth of teaching.” For no kind of knowledge other than the omniscient knowledge of a Buddha can find a footing in it.