Sorry I’m just now seeing this! I’m not sure why that happens exactly, my guess is WordPress thinks it’s the same Ambition theme from their Theme Repository so that’s probably why that happened.
Genesis Child themes don’t usually have Update Notifications, only the Genesis Framework does. So in the future, if you get an update Notification for Ambition, just ignore it in case it’s the wrong theme giving you that alert. When we (or any Child Theme developer) have updates, you usually have to manually update those by deleting/replacing the theme if you need to.
I’ve had 2 other people mention something like this, but it’s never happened to me.