I am discovering it very difficult to realise why the next doesn't work

<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteBase /

    RewriteRule ^category/.+$ category_display.php [L]
    RewriteRule ^index\.php$ - [L]  

    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d 
    RewriteRule . /index.php [L]
</IfModule>

basically comment the second rule (WordPress) it'll work. I had been around the knowning that [L] terminates if your match is located...

Can anybody shine an easy about this?

Ok, just done some investigation and located the solution. Like a test i had been just creating personal files title planning on throw a mistake saying the file doesn't exist. But after reading through I discovered that !-f and !-d determine when the file is available and for this reason it had been tossing the custom wordpress 404.