For an iL870 Stat. 'spend' to outscore an iL900, the 900 must have much less optimal secondaries compared to the 870 - if the two items have the same stat.'s & in the same proportion, the 900 will always be better... I get the impression that primary stat's - while not of major importance to all classes (re; older versions of AMR optimiser) - they still play a part in what's shown (or not) on the stat. graphs we get in this iteration of the optimiser.
That said, the current iteration has way more 'give' in what's good/bad, so it may well be a case of, "What stat's does each have, how much of each & are they worth having in the first place?"
Also, the modifiers on crafted gear get to throw spanners/wrenches in the works with new patches, so @yellowfive may already be on the case with 'curing' it, if that's at play.