Nginx read index.html and index.php with different folders

my nginx config for domain looks like below:

    server {
    listen *:80;


    server_name mywebsite.org ;

    root   /var/www/mywebsite.org/web;



    index index.html index.htm index.php index.cgi index.pl index.xhtml;



    error_page 400 /error/400.html;
    error_page 401 /error/401.html;
    error_page 403 /error/403.html;
    error_page 404 /error/404.html;
    error_page 405 /error/405.html;
    error_page 500 /error/500.html;
    error_page 502 /error/502.html;
    error_page 503 /error/503.html;
    recursive_error_pages on;
    location = /error/400.html {

        internal;
    }
    location = /error/401.html {

        internal;
    }
    location = /error/403.html {

        internal;
    }
    location = /error/404.html {

        internal;
    }
    location = /error/405.html {

        internal;
    }
    location = /error/500.html {

        internal;
    }
    location = /error/502.html {

        internal;
    }
    location = /error/503.html {

        internal;
    }

    error_log /var/log/ispconfig/httpd/mywebsite.org/error.log;
    access_log /var/log/ispconfig/httpd/mywebsite.org/access.log combined;

    location ~ /\. {
        deny all;
        access_log off;
        log_not_found off;
    }

    location = /favicon.ico {
        log_not_found off;
        access_log off;
    }

    location = /robots.txt {
        allow all;
        log_not_found off;
        access_log off;
    }

    location /stats/ {

        index index.html index.php;
        auth_basic "Members Only";
        auth_basic_user_file /var/www/clients/client0/web15/web/stats/.htpasswd_stats;
    }

    location ^~ /awstats-icon {
        alias /usr/share/awstats/icon;
    }

    location ~ \.php$ {
        try_files /b55a31c419b809eb02d1c0a0c8f6e014.htm @php;
    }

    location @php {
      #  try_files $uri =404;
        include /etc/nginx/fastcgi_params;
        fastcgi_pass unix:/var/lib/php5-fpm/web15.sock;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_intercept_errors on;
    }




    location / {
    root /var/www/mywebsite.org/web/mainpage;
                try_files $uri $uri/ /index.php?$args;
    }

}

This configuration is default config with use ISPConfig panel, I've just added "location /" directives becouse my website is in folder /mainpage.

My problem: if index.php is in folder /mainpage then nginx doesn't read it and read index.php one folder upper "/web" BUT if I put in folder /mainpage file index.html then server read it correct… For me this behave it's weird. Why server read correct file index.html in /mainpage but in the same folder index.php no. After delete/rename file index.html in folder /mainpage the server automatically read file index.php one folder upper "/web". Dont want read index.php in /mainpage folder.