[sniperhiga]

Google has many datacenters which collect information from different sources. Each datacenter rank websites different depending on information this particular datasenter has.
Datacenters exchange information with each other on a regular basis and they update their information for each website on 27/7 basis without brakes, lunches, and days off.

Because it is 24/7 process it meant that each website gain value and loosing value all the time. It is like stock market where stocks go up and down. It means that your site could loose and gain some positions on a daily basis.

The most important thing is that in high competitive field websites go up and down really fast.