@LeDiva Starfleet is a very Earth centric member of the Federation.
So, for example while Vulcan is also part of the Federation, outliers such as Spock (half Vulcan half Earthling) are part of Starfleet, while most other Vulcans seem content to simply be part of Vulcan's contingent within the Federation without feeling any need to join Starfleet as part of this.
Does that help? ;)