It looks like what is happening here is when searching for only "Stratford," you are getting results for Stratford, Connecticut. Which - coincidentally - is a similar distance away from Kingston as Stratford, ON.
You can bias your Geocoder results to a specific region using the GEOCODER_REGION option in the plugin, which you can find near the top of geocoder.php. To enable bias for Canadian addresses, you can uncomment the option (Line 21 in the latest versions of the plugin) and use the "ca" region code to look like this:
$GLOBALS['GEOCODER_REGION'] = 'ca';
Give that a shot and let me know if it helps with your issue, or if you have any other questions.