You might as well be asking why each wave you see at the beach looks different: there are literally millions of websites, and most of these are undergoing dynamic changes regularly (is yours?) like common updating, refinements to Content development, Key Meta Rotation, and various other reasons and strategies - each of which directly influences ranking and positioning (and overall www relevancy).....that is why real Webmasters get paid good money, to produce consistent results by continual maintenence of a fluid online dynamic....

It is also why real ethical positioning (or SEO) companies also earn real money, for it literally requires a Team effort to do all the relevancy work and implement it in time to be productive each month. Learn More Here