With the Amazon example, just remember that they don't show you product suggestions for help you, they are showing them to sell more product. If on average the

With the Amazon example, just remember that they don't show you product suggestions for help you, they are showing them to sell more product. If on average the technique works across the entire customer base, then they will apply it. While it would certainly be better if they were more accurate in that fringe case as well, if it would take 4x the effort to eek out a 20% improvement, that might not be a good trade off for them.