为什么我的ORDER BY子句与此ActiveRecord范围重复？

class Coupon < ActiveRecord::Base

  default_scope order(:created_at)
  scope :inactive, where(:active => false)

end

inactive

ORDER BY

> Coupon.scoped.to_sql
 => \"SELECT `coupons`.* FROM `coupons` ORDER BY `coupons`.`created_at`\" 
> Coupon.inactive.to_sql
 => \"SELECT `coupons`.* FROM `coupons` WHERE (`coupons`.`active` = 0) ORDER BY `coupons`.`created_at`, `coupons`.`created_at`\"