Google Question.....
 

How do the keywords start showing up on drop down when you start typing? Is it from whats mostly typed in by majority? or starts adding history? Who knows the true algorithm on this part only....?

Use the google keyword tool. Google that. You will get a google.com link to it.

Then search for something.

Then notice the search volume column.

Now open google and start typing the same keyword(s) and see how they match.

Then it all makes perfect sense...

there are no adult keywords there, all till line are from history other are most popular.

For words that don't correlate in the above example, there is an algorithm that is also checking most often clicked.