Reservation Utilisation Graph / Report

The situation you're describing, where your ElastiCache Reservation Utilization Graph shows 100% utilization but still reports unused hours and associated costs, can occur due to a few reasons:

1. Partial hour usage: Even if your reservations are fully utilized most of the time, there might be short periods (less than an hour) where they're not used. These partial hours can accumulate over time, resulting in unused hours and costs.

2. Size flexibility: ElastiCache Reserved Instances offer size flexibility within the same instance family. If you're using a smaller instance size than what you reserved, it might show as fully utilized, but there could still be unused capacity, leading to unused hours and costs.

3. Reporting granularity: The utilization graph might be showing an aggregated view (e.g., daily or weekly), while the detailed report shows more granular data (e.g., hourly). This can lead to discrepancies between the high-level view and the detailed breakdown.

4. Time zone differences: If the reservation spans multiple time zones, there might be edge cases where usage is counted differently in the graph versus the detailed report.

5. Reporting lag: There could be a slight delay in data processing, causing temporary discrepancies between the graph and the detailed report.

To get a more accurate picture of your reservation utilization:

1. Check the "UnusedQuantity" and "UnusedRecurringFee" fields in your detailed Cost and Usage Reports for the specific reservation line items.

2. Look at the "reservation/UnusedQuantity" and "reservation/UnusedRecurringFee" columns in your AWS Cost and Usage Reports (CUR) for more granular data.

3. Consider the time frame of your analysis, as shorter periods might show these discrepancies more prominently.

4. If the issue persists or you need further clarification, it's advisable to contact AWS Support for a detailed analysis of your specific case.
   Sources
   Understanding unused reservation costs - AWS Data Exports
   Reservation details - AWS Data Exports
   Reservation columns - AWS Data Exports