Google search results changing with page #?

What do you make of the following google search results changing with page number ? Note the result page and the total number of pages shown below ... Why does it change with every page ? and that too so drastically ? going up and down !!

If you know the answer, please let me know ! Of course, there is some dynamic decision making going on in there - but whats it ? and are we sure that the 5th page (link to thre 5th page) mentioned on page 1 is the same page as the 5th page link mentioned on page 4 ?

btw, If I click on page 10 link on page 1 - I get the 5th page only !! so I think this is a bug ... Known or unknown thats the issue :) ...

Update : I dont see this behaviour on the www.live.com (Microsoft's online search engine). It shows you the exact same pages all the time !