>If the item you're ordering is out of stock or unavailable to ship immediately, the shipping method time starts when the item ships. For example, it will take two business days after an item ships to reach you with Two-Day Shipping.
Simply put - the item does not ship immediately. If you read the 'Guaranteed delivery date' message it gives you an "If you order in the next x hours and y minutes". That is when the shipment is expected to be made.
Why are the boots 4 day shipping? Because the next guaranteed delivery time is in 61 hours (>48 hours, so 2 days to ship then 2 days to deliver = 4 day delivery). If it was "in the next 73 hours" it would be a 5 day delivery.
The exploding kittens is 3 days because the order is within the next 24 hours. I wager if they waited 17 minutes one of two things would happen: it would estimate 2 day shipping or the date would be pushed back an additional 24 hours (never falling under 24 hours).
I imagine this is because the companies selling these items can only guarantee shipment within 24/48/72 hours of receiving an order. If they set this to 72 then "2 day shipping" is really 4-5 days.
My personal experience, nearly all of my 2 day shipping comes within 2 days, even when the guaranteed date is 3-4 days out. Items with 12-15 business day shipping often arrive in just a few days. I'm a bit forgiving when my 2 day shipment comes in 3 days when my 2 week shipments frequently come in 3-5 days...