Cost of item = 10

  First discounted cost of item = 9
  => First saving = 1

  Second discounted cost of item = 6
  => Second saving = 4

  Second saving is 4x first saving.

  (Edit - formatting)

But that's 4x the savings compared to another saving. I suppose you've upped the pedantry and are technically correct, but that's a pretty narrow use case and not the one used in the article.