ttavoy the SPF record defines the IP addresses that can send mail for your domain. In this case those IPs are defined in an included TXT record for the hostname "_spf.google.com".
There's no requirement that the domain of the SPF endpoint (_spf.google.com) and the hostname of the SMTP server must match, all that matters is that the IP of the sending server appears somewhere in the SPF record or in one of its included endpoints.
