Jekyll collection returns the wrong permalink in plugin

I have a jekyll collection that looks like this:

collections:
  docs:
    output: true
    permalink: /:categories/:name

and one document that overrides that permalink:

---
layout: default
permalink: /about_us
title: about
tags:
 - about
 - docs
---

but when I try to output the permalink at the event of site generate in a plugin

  module Jekyll
  class DuplicateCollectionGenerator < Generator
    safe true

    priority :highest

      def generate site
        site.collections['docs'].docs.map do |article|
          puts article.url
        end
      end
    end
  end

I get the wrong one, I get one called /legal/about_us

Why is this happening and what can I do about it?