Why are devices listed when I use Account Key to sign in?

When you enable Yahoo Account Key to sign into your account, the account gets associated with your push-enabled mobile device. You can associate more than 1 device per account.

During a security review, you'll see a list of associated devices. This lets you know what devices currently have access to your account. Yahoo strongly suggests you sign out of unfamiliar, old, or unused devices.

Clicking sign out will disconnect that device from your account and it will no longer receive sign in notifications.

You can also check your recent activity to see and sign out of devices and browsers that are connected to your account at any time.