Is my code buggy or does prawn have a bug?

sheet.rb

require 'rubygems'
require 'prawn'
require 'prawn/measurement_extensions'

class Sheet
  SIZE   = "LETTER"
  LAYOUT = :portrait

  ELEMENTS = {
    :header  => { :position => [(4.5).in, (10.0).in], :width =>  3.in },
    :address => { :position => [(0.0).in, ( 8.5).in], :width =>  3.in },
    :method  => { :position => [(4.5).in, ( 8.5).in], :width =>  3.in },
    :table   => { :position => [(0.0).in, ( 6.5).in], :width => (7.5).in },
    :footer  => { :position => [(0.0).in, ( 1.5).in], :width =>  3.in }
  }

  class << self

    def multiple(orders, pdf = nil)
      first = !pdf
      pdf ||= Prawn::Document.new(:size => SIZE, :layout => LAYOUT)
      orders.each do |order|
        pdf.start_new_page unless first
        new(order, pdf).render
        first = false
      end
      pdf # returns the prawn pdf instance
    end

  end

  attr_reader :order, :pdf

  def initialize(order, pdf)
    @pdf    = pdf
    @order  = order
  end

  def render
    [:header, :address, :method, :table, :footer].each do |element|
      settings = ELEMENTS[element]
      pdf.bounding_box(settings[:position], settings) do
        self.send(element)
      end
    end
    pdf # returns the prawn pdf instance
  end

  def header
    pdf.float do
      pdf.text "Invoice #"
      pdf.text "Date:"
    end
    pdf.text @order[:receipt], :align => :right
    pdf.text Time.now.strftime("%b. %d, %Y"), :align => :right
  end

  def address
    pdf.text "Ship To"
    pdf.text @order[:name]
    pdf.text @order[:addr]
    pdf.text @order[:csz]
  end

  def method
    pdf.text "Shipping Method"
    pdf.text @order[:ship]
  end

  def table
    pdf.text "Ordered Items", :align => :center
    data = @order[:items].map do |item|
      [
        item[:sku],
        item[:qty],
        item[:desc],
        item[:price],
        item[:total]
      ]
    end
    pdf.table data.unshift(
      ["SKU #", "QTY", "Description", "Unit Price", "Total"]
    )
  end

  def footer
    pdf.text "Filled By:",       :align => :right
    pdf.text "Checked By:",      :align => :right
    pdf.text "Processed On:",    :align => :right
    pdf.text "Tracking Number:", :align => :right
  end

end

OUTFILE = "output.pdf"

orders = []
(1..20).each do |i|
  orders << {
    :receipt => i.to_s.rjust(4, "0"),
    :name    => "Bob Jones #{i}",
    :addr    => "#{i} S. Main",
    :csz     => "Hometown, USA",
    :ship    => "EXPRESS",
    :items   => []
  }
  (1..5).each do |j|
    orders.last[:items] << {
      :sku   => (1000 - i * j).to_s.rjust(4, "0"),
      :qty   => j,
      :desc  => "Fab. product #{j}",
      :price => "$1000.00",
      :total => "FREE!"
    }
  end
end

pdf = Sheet.multiple(orders)
pdf.render_file(OUTFILE)
puts "Done."