google showing wp-login.php in results

I looked through the google results you linked to, and do not see a link to that sites login page — that said I didnt go through every result, so I’ll take your word for it.

why this is happening?

Thats why I looked through the google search results. The why is very simple — at some point, possibly while there was another theme running on that site Googlebot followed a link to wp-login.php.

Thats the way spiders work — they follow links.

if you dont want that page OR other pages indexed, use a robots.txt file.