Google started out by building a search engine based on automated algorithms. Clean, comfortable, don’t have to deal with people – just what the doctor ordered for nerds.
However, people are smarter than algorithms and any bogus AI, so they figured it out. They are the cats in the cat-and-mouse.
What’s poor Google to do?
They can apply human input (as they already do now). But it’s not really scalable, it’s unreliable, it’s messy – and webmasters, marketers and spammers will figure this out, too.
One thing people cannot figure out is randomness.
So they put randomness in the algorithm; basically retrofitting the most critical and most vulnerable elements of their ranking algo. (In other words, the important yet easy-to-reverse-engineer parts.)
And this also explains neatly why the quality score for AdWords behaves in such irrational and unfathomable ways at times.
It has even more built-in randomness than organic rankings. It needs to. Why?
Because, compared to the organic SERP, here the feedback users receive is more instantaneous and more structured, the scope for user experimentation is greater (and the incentive to do so is more obvious), therefore the potential for gaming the system is much larger.
However, randomness built into the organic results is not much of a problem in terms of relevance. Searchers won’t notice, or care about, anything odd as long as the SERP remains acceptably relevant.
At the same time, AdWords data are closely monitored and analyzed, sliced and diced, so the randomness is more observable. It’s more disturbing and it hurts budgets.
In the end, it has the potential to alienate the very people Google makes a (very good) living from.