I have seen 2-3 sites are always at top one or within the top 5 for some of the keywords. How it is possible? Why should not my site get top one for that particular keyword ranking.

Page rank does not depends only on keyword but also content and backlink plays very important role in increasing the ranking of your page.

There is lots of ranking factors not one so optimize your website on that keywords and build links on particular keywords.

There are many reasons such as
-The sites are very well optimized with a number of good quality backlinks
-The sites have very high Pageranks
-The sites are very old and they look like authority sites in the eyes of Google.