The key difference is it is hosted directly inside your Moodle instance. Speed tests will always be specific to the particular combination of the end users computer, network, and all of the hops in the middle up to and including the actual moodle. No 3rd party test will ever give you a true indication of exactly how fast a user can access this specific moodle site.
In fact using a test tool such as Google's which will likely be hosted on CDN edge servers very close to your user will almost always give a false and overly optimistic estimate of the true network speed to your moodle instance.
SUPPORT: This plugin page and forum is NOT regularly monitored!
If you have any issues or need support please use github or contact Catalyst IT Australia:https://github.com/catalyst/moodle-tool_speedtest/issues