I once called and spoke directly with their tech department, and was told that the OE products were "inferior" (their word, not mine) to the other Amsoil offerings. Now - that most admittedly is a loaded statement sure to illicit much chagrin from almost any Amsoil lover. I will, however, say that I did NOT take it that Amsoil was inferior over-all; just that the OE products were not as robust as their other premium typical offerings. That conversation took place a couple years ago, not long after the OE motor oils came out. I was specifically told that the OE products were very capable and would well withstand the OEM OCIs (their target market), but that products were not warranted for more than that, and therefore not designed to go longer. I interpreted that to say the base stocks were lower and add-packs lighter. I was not told that directly, but when I asked, I was not discouraged from making that conclusion; the answer became evasive at that point.
I'm guessing here, but I would suspect the base stock is group III and not a IV/V? It's probably similar, conceptually, to their OE motor oil offerings. The add pack would also not need to be as robust; OEM OCIs really don't call for much in the way of "above and beyond" supplemental package bolstering.
It's not that the OE products are not good; they probably are every bit as capable as other typical syns. They are marketed to OEM OCIs, and therefore simply don't have a very high bar to jump over. That being the case, I suspect both the base stock and the add-pack are not as strong as their more premium fluids, and therefore holds the cost down. So, to be fair, I cannot tell you why the costs are less with OE fluids, but then I dont' have to be a genious to guess ... especially when I'm told they are "inferior" relative to the premium offerings, by the very folks that make/market them.
I always encourage folks to go right to the horse's mouth, and hear it for themselves. I always call the maker directly, if I can. But Amsoil will become somewhat tight-lipped when you try to nail them down on specifics; as would just about any other lube maker.