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Google has far more information available about these searchers than we do and I suspect it is capable of making a pretty fair evaluation (based on previous actions, etc.) about how "hot" these leads are. It could meter them out to various merchants, sending the hot ones one direction for a while, etc. But why? And why alternate over a multi-day period?
What I find difficult to believe is that people do not grasp it is an algorithm. There aren't people sitting in a factory pulling little levers either approving or disapproving a given result.
Google has automated taxonomy assignments for query types and for URLs. If the taxonomy assignment algorithm can't figure out where your page "fits", they might keep trying different categories for it in an automated, machine learning fashion.
Could it be that Google refuses to realize that a domain has both informational and commerce pages?
There are so few people reported this kind of traffic that I wonder how wides"pread it really is.