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.
An all-time average will always slow to a crawl. The longer the period is the less of an impact newly found blocks will have. I included it in my chart in the beginning, when there wasn't enough data for a 7-day average, I'm thinking of removing it now because it isn't very useful.