Amazon uses machine learning for more of the site than ever. Machine learning aims to provide tailored shopping experiences to individual accounts. The original Kindle store was as straight-forward as we were ever going to get . . . and over the years Amazon has made changes to overcome rank manipulations, better align to their business goals, appease large vendors, and improve the experience for the customer.
Check your own site. I suspect more than 50% of your traffic is mobile. Now look at the mobile site of Amazon. You will see that it provide not even a WAY for readers to get onto the top 100 FREE list. Try it with your phone. Even if you go to the bottom and click "amazon.com full site" go to a book that's ranked top 100 free and try to clik on the link to top 100 free, it takes you to the top 100 paid. Do a drop down and you can only get top 100 paid, top 100 new release. I sat at the bar at NINC and realized this with T S Paul who was like "the Amazon people are right there, you can ask them about this . . ." because we both wondered if it was a glitch. It's 1.5 months later still doing that (and no I didn't bother the Amazon people with this), it's NOT A GLITCH.
What Patty and others I think are trying to say is that if you build your entire business strategy on getting sticky at a single vendor there will come a time when changes to their system impact your publishing empire. Right this second, look around, we are in the middle of a major change to Amazon's systems that are derailing many people's marketing campaigns. Spikes are out, somehow, and we've watched Amazon punish spikes to varying degrees over the last 5 years.
There's no way to predict from what Amazon is doing right now to what they will be doing in 1 year or 2 years. Who knows when KU 3.0 or 4.0 or whatever number we are on will be out? Who knows when/if Amazon will separate out the KU books into a separate sales ranking so their A Pub titles can dominate in two powerful recommendation engines? Who can tell everyone exactly how Also Boughts are deterined because mine are most frequently new releases in my genre, not my other books, which make NO SENSE. . . .
All efforts to publish based on leveraging the Amazon algorithms will have to be reactionary in nature, and it's basically a game of musical chairs. Something works until it doesn't, because the music stopped and we didn't know and now people are left without a chair. Some publishers shrug and go "OK!" and expect some launches to flop or some promotion to fail and just keep on going and kept a budget thus. Others can't or won't do that and hinge every major release on a wish and prayer that all of the stuff they took months to put into place WILL work the day of launch...Others just go "eh, I'll let Amazon worry about Amazon, and just focus on selling books any way I can."
I read Patty's post as publishing off of the current Amazon algorithms is unpredictable because it requires assumptions and observations where we are most often trying to justify the phenomenon we see. It's unknowable because we aren't in the loop on when or why changes are made, and unless someone is prepared to rise up from when the algorithms change and they weren't ready, it can be very demoralizing and devastating if you are counting on a specific performance of a book to say feed your family or pay your bills.