Oops, I see what I did. I was comparing digital subscription revenue to overall ad revenue. Comparing digital subscription revenue to digital ad revenue is "only" a factor of 8.8 rather than 18 (because just over half the ad revenue is print).

Note that newspapers make little if any profit from print subscriptions in themselves, because they're little better than breakeven with the cost of printing and delivery. The real profit has always come from the advertising. Digital subscriptions are something new in that they're actually making money from subscription revenues instead of just using them to cover unit costs, but they're currently still tiny.