That you get Pizza at all is a freakin' miracle!
"Millions of deliveries" with a "five or six nines" 99.999% or 99.9999% success rate would almost certainly be considered spectacularly successful by Domino's. Wikipedia says $1.8billion in revenue, 220,000 employees, and 10,000 stores - some outrageous assumptions might be made. Lets say ~$500 per day per store is, what, a 15% of revenue franchise fee as a guess? So a store on average makes $3-4k/day? So maybe 3 to 4 hundred pizzas a day, right? Five nines would be one error per store per year. Six nines would be one error per store per _decade_. I bet their real-life error rate is _way_ worse than that.
But, across 10000 stores and 220,000 employees, a billion or so pizzas a year isn't really a very big number.
Now consider Google, or Facebook, or Amazon. How many emails/adviews/status-updates/likes/widget-orders do you suppose happen every hour? Would it be as low as 1 billion per hour? And would a spectacular seeming "six nines" of reliability, which means dropping a thousand mails/adviews/updates/orders _per hour_ be acceptable?
"failures from time to time" in some contexts, can mean "fundamentally and irrevocably broken" in other contexts...
On the other hand, after few years on various logistics-related projects, I would not be too surprised if there is some postal service or carrier that is actually able to somehow physically duplicate one shipment and deliver it twice :)
More than once delivery in real-life :)