Why the Allocation ID and Association ID exist for AWS NAT Gateway and EIP

Let me ask you a question about AWS network resources.

There seem to be two relationships between NAT gateways and EIP: Allocation ID and Association ID, but why are there two?

I was recently organizing AWS resources, and when I was following the floating EIP, I eventually got to the NAT gateway.

EIP
↓ Association ID
NAT-owned ENI
↓ Attachment ID
NAT Gateway

As I followed up, I saw the above relationship, and besides the Association ID that connects NAT and EIP through ENI, there was an Allocation ID that seemed to connect NAT and EIP directly.

When you create NAT, it automatically generates ENIs, and I don't know why there are two separate ones.

Is there something wrong?
Or is it a trace of AWS' evolution?

Thank you for your cooperation.