The presence of unused hours for your EC2 Reserved Instances (RIs) in AWS can stem from various factors that might not immediately be apparent. One possible reason is that the type of instances you’re launching doesn’t match the ones you’ve reserved. For example, if you purchased a reserved instance for a certain instance type in a specific Availability Zone and are instead launching different instance types or in different zones, those hours will be counted as unused. Additionally, the purchased RIs are only valid for the regions and instance types specified during the reservation, so misalignments in your deployment strategy could lead to significant unused capacity.

Another consideration is the potential variability in your workload requirements. If your applications have fluctuating usage patterns or if you’re utilizing Auto Scaling, it’s possible that the dynamic nature of your workloads results in underutilization of your reserved instances. In such cases, while you may have reserved capacity to handle peak loads, actual demand may not always match those peaks, leading to a situation where you’re billed for resources that remain idle. To maximize the benefit of your RIs, regularly review your AWS usage reports, and consider adjusting your reservations periodically to align more closely with your current usage patterns if you find that you consistently have unused hours.