Gravatars are based on email addresses. No matter what sitewebsite you're on, if it supports gravatars and you enter your email address, it just works!
Adding e.g. Super User account ID or display name to the gravatar resolutionidentifier (hash of email address) will only break custom gravatars for users registered with the service who have a custom image.
So if a user has multiple accounts and uses the same email address for these accounts, all of the accounts get the same gravatar (and there are worse). That's simply how it fundamentally works. It's a feature, not a bug.