Rails - May 2012 - Why does Rails only save the joiner record in one instance but not another?

Assuming we have User, Account, and Role models, and a User has_many 
:accounts, through: :roles and vice-versa, with the following snippet Rails 
automatically creates the Role (joiner) record, but with the snippet after 
Rails will not. I think I''m misunderstanding how has_many through
works, so
I would appreciate any insight into the subject.
 

def new
  @account = Account.new(params[:account])
  @user    = @account.users.build
end
def new
  @account = Account.new(params[:account])
  @account.save
end

Using this snippet Rails *does not* save the Role record: (If you replace 
current_user.accounts.build with current_user.accounts.create Rails will 
save the Role record)

def new
  @account = current_user.accounts.build
end
def create
  @account = current_user.accounts.build(params[:account])
  @account.save
end

Models:

class User < ActiveRecord::Base
  has_many :roles
  has_many :accounts, through: :roles
end
class Account < ActiveRecord::Base
  has_many :roles
  has_many :users, through: :roles
  accepts_nested_attributes_for :users
end
class Role < ActiveRecord::Base
  belongs_to :users
  belongs_to :accounts
end

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