well there is still a difference. Under the 100 page plan, you can print 100 completely black pages using the max amount of ink for the plan

or you could print 10 pages with a single sentence in them

HP sends you new cartridges depending on your use. So the second person in my example would not get a new cartridge, while the first would get 3 in the mail the next day. Both are paying the same monthly payment.

Its a dumb idea, but some people opt-in to it. And from my personal experience people end up with lots of extras being shipped to them. They can have 5-6 full cartridges at home at a time, for future use. HP doesn't want you to get stuck without ink, so they overstock you. Cancelling can cause people to take advantage of that overstock