I want to understand why some of the line items on my AWS bill have the "blended" annotation on them.
For billing purposes, AWS treats all accounts in an organization as if they're one account. The pricing tiers and capacity reservations of the accounts in the organization are combined into one consolidated bill. Combining the accounts into consolidated billing can lower the effective price per hour for some services.
This effective price per hour is displayed as the "blended" rate in your Cost and Usage Reports or AWS Cost Explorer. The blended rate that's shown in the console is for informational purposes only. For more information, see Blended rates and costs.
An organization's management account can restrict the pricing benefit of Amazon Elastic Compute Cloud (Amazon EC2) Reserved Instances to the accounts that they’re purchased on. To restrict the pricing benefit of Amazon EC2 Reserved Instances, see Turning off shared Reserved Instances and Savings Plans discounts.
What is AWS Billing?
Understanding consolidated bills