Say most of your site visitors come at 1pm, Google will focus on giving your site more impression the next day at around the same time. Say the response is the same, Google will keep repeating that. But say people are not as responsive next week, and more people seem to come to your site at 4pm now. The same process repeats and your impressions increase during 4pm.

Or at least, that is what I think. I could be completely wrong.