Verifying NAU metric calculations for VPC

Your understanding of NAU calculation for VPC endpoints is correct, but there might be some nuances to consider in your specific case.

Generally, a VPC endpoint contributes 6 NAU units per Availability Zone (AZ) it spans. With your VPC endpoint spanning 6 AZs, you would indeed expect a minimum of 36 NAU units from this resource alone.

However, there are a few possibilities to consider:

1. Timing: NAU metrics are typically reported every 24 hours. If you've just enabled NAU metrics, it's possible that not all resources have been accounted for in the first metric report. You may need to wait for the next reporting cycle to see a more accurate count.

2. Resource state: While you mention that all the ENIs are active, it's worth double-checking the state of your VPC endpoint and its associated network interfaces. Only active resources contribute to the NAU count.

3. Resource types: Different AWS resources contribute different amounts to the NAU count. For example, EC2 instances contribute 1 NAU per private or public IP address assigned to their network interfaces. It's possible that other resources in your VPC are contributing to the NAU count, but not in the way you expected.

4. Edge cases: While not explicitly mentioned in the documentation, there could be edge cases where resources are assigned different weights. However, this is not typically the case, especially for well-established resources like VPC endpoints.

To get a more accurate picture, you could:

1. Wait for a few reporting cycles (a few days) to see if the NAU count stabilizes at a higher number.
2. Manually inventory your VPC resources and calculate the expected NAU count based on the documentation.
3. Contact AWS support for a detailed explanation if the discrepancy persists.

It's understandable to be cautious given past experiences with false positive metrics. Continuing to monitor the NAU metrics over time and comparing them with your manual calculations will help build confidence in the accuracy of these metrics.
Sources
Network Address Usage for your VPC - Amazon Virtual Private Cloud
Amazon VPC quotas - Amazon Virtual Private Cloud
CloudWatch metrics for your VPCs - Amazon Virtual Private Cloud