Postgres RDS is indeed different, yes it runs on EBS, but the Aurora in a sense isn't truly Postgres. What they've done is forked away from vanilla Postgres and

Postgres RDS is indeed different, yes it runs on EBS, but the Aurora in a sense isn't truly Postgres. What they've done is forked away from vanilla Postgres and tweaked the storage layer to work on the block based storage they've built just for it. It's why you see it advertised at "Postgres compatible" vs. just being will standard Postgres. This article is definitely an interesting one as it highlights some of those trade-offs that they now need to account for.