I'm sure they crawl them, and always have. If the URL is example.com in the case you describe, it is quite likely that the page is receiving page rank from both internal and external sources, compensating for poor on page content.

Did you check the meta tags? It may be that your search term brought out the best in that URL!

but it is not the index page.moreover it has no content and automatically directs to index page within 5-10 secs.

Someone's gone to some lengths to get the page indexed .. there's some strange folk about!

The only sensible explanation is that the URL previously held a proper page, and was recently changed, however I suspect it's not a sensible explanation that will do! - but without the detail, there's no way to find out exactly what's occurring.