ruby - Delayed Job gem を使用した定期的なタスク

Question

class A < ActiveRecord::Base
  def self.a
    A.first.touch
    delay(run_at: Proc.new{ 10.seconds.from_now }).a
    log.info 'Added job to run at ' + 10.seconds.from_now.to_s
  end

  def self.log
    @log ||= Logger.new 'dj.log'
  end
end

ワーカー ログに次のように表示されるのはなぜですか。

Added job to run at 2013-04-26 01:04:53 UTC
Added job to run at 2013-04-26 01:04:53 UTC
Added job to run at 2013-04-26 01:04:54 UTC
Added job to run at 2013-04-26 01:04:54 UTC
Added job to run at 2013-04-26 01:04:55 UTC

注: 10 秒間隔ではありません。