Nuances with member routes.. why does a leading slash override the helper method?

I recently just ran into this issue and am curious as to why this happens… I have a resource and went to add a new member route onto it. I ended up with something that looks like this:

resources :wikis do
  member do
    get 'new', to: 'wiki_pages#new'
  end
end

After this, one of my urls broke. This helper:

wiki_path(generation.wiki_id)

Went from returning /wiki/1 to /wiki/1/new… but! If I add a leading slash it doesn’t override the helper method. This is currently working as expected (helper method returns /wiki/1)… but why?

resources :wikis do
  member do
    get '/new', to: 'wiki_pages#new'
  end
end