tangor - 5:08 pm on Jan 13, 2012 (gmt 0)
Estimated is just that, an estimate. Historical data is generally fixed (known) and can be useful in some revenue planning (or trend tracking). One also needs to include "other world" statistics to see if there is correlation, such as rise/fall in banking, markets, consumer trends... particularly from target audience. Include ordinary cyclic trends, consumer demand, shifting markets, changes in product offerings, etc.
Could simply be the widget of yesterday is not performing as well today.
Sometimes (ahem) it might NOT be google which is responsible.