I think the X factors in the specific scenarios you're describing are probably mainly related to the number of accounts with balances. $10 on one card, $5 on another card, and $0 on the other two cards is not exactly the same as $15 on one card and $0 on three cards.
I had a confusing situation recently where it turned out that my score had dropped ~15 points because my non-AU credit cards were all at zero; obvious in retrospect, but it confused me because I thought the AU credit card and/or my charge card (both of which showed balances) counted for that calculation.
I think these small fluctuations in balances have too much effect on scores, and sometimes, an effect that I think is opposite of what it should be (but just my opinion). I don't see a reason that having a $10 balance on one card with an aggregate CL of (say) $50,000 should result in a significantly higher score than having a balance of $0 with the same CL. I am interested in learning more about FICO 10, which may smooth this stuff out at the cost of sabotaging some strategies like AZEO.
Also, I think FICO 8 should weigh credit age and recent inquiries much more heavily, because lenders weigh that heavily. This is something on my mind, because despite my pretty good scores, I would be rejected for a lot of "average" cards and loans because of these factors (for example, I don't know if I'd be approved for a Citi AAdvantage card, I think maybe not). That tells me that FICO 8 doesn't serve the market's needs if lenders routinely have to do their own calculations instead of simply using using the score.
