I think you're over thinking this. Reads from regular readers are worth much less than new or occasional readers. You probably just picked up more new or occasional readers on the higher value story. You don't get paid for non-member reads at all, so that's a red herring. I've written some articles with breakdowns that show exactly what's happening, recently.

BTW, I thought you might be interested in my article about being an imperfect vegan. :) https://medium.com/age-of-empathy/no-were-not-proper-vegans-you-re-right-7b812ab37229