Boolean argument error in Ruby

I am working on POODR's chapter 8, Combining Objects with Composition. However, I can't seem to make the example work:

class Bicycle
  attr_reader :size, :parts

  def initialize(args = {})
    @size  = args[:size]
    @parts = args[:parts]
  end

  def spares
    parts.spares
  end
end

---

require 'forwardable'
class Parts
  extend Forwardable
  def_delegators :@parts, :size, :each
  include Enumerable

  def initialize(parts)
    @parts = parts
  end

  def spares
    select { |part| part.needs_spare }
  end
end

---

require 'ostruct'
module PartsFactory
  def self.build(config, parts_class = Parts)
    parts_class.new(
      config.collect do |part_config|
        create_part(part_config)
      end
    )
  end

  def self.create_part(part_config)
    OpenStruct.new(
      name:        part_config[0],
      description: part_config[1],
      needs_spare: part_config.fetch(2, true)
    )
  end
end

---

When I use road_config:

road_config =
  [['chain',        '10-speed'],
   ['tire_size',    '23'],
   ['tape_color',   'red']]

road_bike =
  Bicycle.new(
    size:  'L',
    parts: PartsFactory.build(road_config)
  )

p road_bike.spares.size

road_bike.spares.size returns 3. Which is what I expected. However, when I use mountain_config:

mountain_config =
  [['chain',        '10-speed'],
   ['tire_size',    '2.1'],
   ['front_shock',  'Manitou', false],
   ['rear_shock',   'Fox']]

mountain_bike =
  Bicycle.new(
    size:  'L',
    parts: PartsFactory.build(mountain_config)
  )

p mountain_bike.spares.size

I was expecting to see 4, not 3. If I delete false from front_shock to let it to the default value (true), mountain_bike.spares.size returns 4.

Why false makes mountain_bike.spares.size return 3? What am I missing?