Automatic User Registration Problem

Hello,
first i don’t know in which category goes so sorry for that.

My problem is i started a new site. It is still in construction.
I even not added any login or registration for in it.
And there is no restriction to become member to read the posts of my site.
Still there is 3-4 registration completed.
How it is happened is this any error…..??
I don’t know about it.
Please any expert tell me.

Site – http://www.studyapi.com

Thanks.