Ruby increment operators not working like we think they're working?

gistfile1.rb

# Stub class acts as a proxy for the memcache Rails cache. Stores key/value pairs in files.
class FakeCache
  def self.write(key,value)
    File.open(key,'w') { |f| f.write(value) }
  end

  def self.read(key)
    begin
      File.open(key).each_line{ |s| result = s }
      result
    rescue
      return nil
    end
  end
end

# Based on our Restaurant model
class TestOp
  def self.cache_key=(num)
    #Rails.cache.write('restaurant_cache_key', num)
    FakeCache.write('restaurant_cache_key', num)
  end
  def self.cache_key
    #unless r = Rails.cache.read('restaurant_cache_key')
    unless r = FakeCache.read('restaurant_cache_key')
      r = rand(10000)
      FakeCache.write('restaurant_cache_key', r.to_s)
    end
    return r.to_i
  end
end

# start w/ a clean slate
File.delete('restaurant_cache_key')

# Try to read a nil cache, increment it, read again.
puts TestOp.cache_key
TestOp.cache_key += 1
puts TestOp.cache_key

# Result
# ~/_work/elctech/temp> ruby testop.rb
# 130
# 5912
# ~/_work/elctech/temp> ruby testop.rb
# 4044
# 9614
# ~/_work/elctech/temp> ruby testop.rb
# 8176
# 7017
# ~/_work/elctech/temp> ruby testop.rb
# 296
# 5709
# ~/_work/elctech/temp> ruby testop.rb
# 2074
# 4293