AWS RDS instances are not using the latest generation of instance classes for cost and performance improvements.
In AWS Console -
In Terraform -
References:
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Concepts.DBInstanceClass.html#Concepts.DBInstanceClass.Changing
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/rds_cluster_instance#instance_class