Now I checked Keyword ranking for a website. But I get some difference between Google and Yahoo ranking. Some keywords are in top 10 in Google, but in Yahoo and Bing these are not in 100. What is the cause in keyword ranking variation in Google and Yahoo?
Each search engine has its own algorithm. Because each search engine uses its own algorithm, you'll see variation in ranking between Google and Yahoo.
Simply because different search engines have different search algorithms so the results are likely to be different.