I think the term is "reversion to the mean".  

Honestly not sure why the author chose these timeframes because they do nothing but add confusion.  A 30 day, 7 day, and 1 day graph would be more useful. Maybe add in a "since last dividend" and "since start of mining" column.

ASICMiner's historical hashrate (all blocks over all time since the start of mining) has only averaged ~30 TH/s.  This lifetime average has flatlined (growth slowed to a crawl) over the last couple weeks.  Not sure why but, you only get paid on what is actually mined.