In truth - no one really knows. Below are all teh details amazon state around supporting and appealing an a - z claim. They clearly state that if it is unclear from other tracking information the item was delivered (you know, like GPS and photos) they will refer to a signature. You’ve got the lot - so who knows why they don’t believe its been delivered.

1. Shipping

• Include valid tracking for all deliveries: Tracked shipments allow customers to see the status of their delivery without needing to contact you. Amazon will grant a delivery-related claim in the customer’s favor if you have not provided tracking details. For more information, go to Valid Tracking Rate (VTR) and Order Defect Rate. You can edit order tracking information by going to Orders, and then Manage Orders. Enter the Order ID into the Advanced Search. Once you find the correct order, click Edit Shipment and provide the revised tracking number.
• Use carriers that can provide proof of delivery upon request.
• Require signature confirmation: A signature helps Amazon determine if a customer has received their item(s). If it is unclear if an item has been delivered (for example, if we cannot establish via other tracking information that the item was delivered to the customer), a customer’s claim is more likely to be granted which is why a signature confirmation is helpful.
• Confirm shipments before expected ship date: Make sure the courier picking up deliveries first scans the customer’s package(s) on or before the ‘ship by’ date. It is important to confirm that you have sent orders on or before the expected ship date so that customers can see the status of their order and understand when to expect delivery without the need to contact you. Failure to ship on time negatively impacts your Late Shipment Rate and makes it more likely that a Claim will be granted in the customer’s favour.
• Ensure delivery is completed before the Estimated Delivery Date: Notify the customer if a delivery is delayed.
• Use supported couriers: Amazon offers Buy Shipping Services that enables sellers in certain stores to purchase pre-paid, tracked shipping labels from Amazon and have packages collected directly from their premises. If sellers ship on time using Amazon’s Buy Shipping, sellers are protected against A-to-z Guarantee Claims if the Claim is related to delivery issues. Under those circumstances, even a granted Claim will not impact your account health, although you may still be responsible for reimbursing the buyer.