I can't the register page find a page in my website


I have a page named “register” on my website (www.positifstyle.com/register/). This page is available on the front end of the site. Its a default page, I never create it.

Now, I’m want to customize it but I can’t find it. Its not in my pages list or in membership plugin pages… I can access to all pages and edit them except this one. How can I access to this page a edit it?

Best regards,