Amazon track deliveries.

See your delivery performance statistics.

Most likely the courier/s you are using have delivered late too many times.

If you shipped on time, Amazon will not send you a violation, as you actioned ok, BUT, they will still be allowing more than 1 day to deliver, that is shown to them when Buying, not just on the confirmation.

If you have a competing Seller, you will lose the Buy Box though, so very unfair.

So, Seller A shows a delivery tomorrow, Seller B (you) show as 2 days, and lose many orders.

We had an issue when DPD did not turn up one day and then all orders were being shown as Prime 2-day deliveries. It righted itself a few days later when virtually all were delivered the next day again.

If you use more than 1 courier out of Amazon, Evri, DPD and Royal Mail, see which is under performing, and then use the other/s.

Annoyingly, sometimes Amazon make us use DPD at a higher cost as they think Royal Mail is under performing to the Buyers Postcode, somehow Amazon also seem to think Evri are ok, when they tend to be the worst for being late.