Routing resources with and without member blocks

Normally, I declare Rails routes to actions for singular resources in a member block as follows:

resources :blog_posts do
  member do
    post 'tag'
  end
end

this causes rake routes to produce the following:

tag_blog_post POST   /blog_posts/:id/tag(.:format)  blog_posts#tag

But today I came across a situation in which the action for a singular resource was not in a member block:

resources :blog_posts do
  post 'tag'
end

which caused rake routes to produce this instead:

blog_post_tag POST   /blog_posts/:blog_post_id/tag(.:format) blog_posts#tag

There are two important differences in the produced routes:

• When in the member block, the resource is identified simply by the :id parameter, whereas without the member block a more verbose :blog_post_id parameter, which includes the model name, is required.

• When in the member block, the action comes at the beginning of the route helper (tag_blog_post) whereas without the member block it is prepended (blog_post_tag).

The differences are interesting. I wonder if there is a reason why they operate this way, or if it's simply a curious side effect of the implementation.