No two search engines will give same result until one search engine results are driven by the other. Bing is a completely different search engine from Google, Bing's algo for web search results and Google algo for ranking in SERPs are different. http://www.open-source-development.com/
Bing has different Algorithm compared to what Google uses... I know Google's results is much better for me (based on my observations).
they give different results because they are different search engines with different search algorythms
They are using different algorithm because they must behaving a newer, different opinion on how search results should be.