The issue appears to be related to the setup of your website, as I didn’t manage to replicate the problem on my test environment.
I suspect that some of your other website components interfere with the plugin. What I can suggest you to try, is to leave our optimization plugin active and disable all of your other WordPress plugins and then run another speed test. If that fixes the issue, you can re-enable your plugins one by one, testing as you go, until you find the exact plugin that interferes with SiteGround Optimizer. You can then try tweaking the settings of the faulty plugin in order to find which exact feature is causing the problem. If the problem still persists, even after all of the plugins are disabled, you can try to temporarily switch the website theme to the default one and run the tests again.
If you are a SiteGround customer, my recommendation is to submit a support request from your Client Area, and we will further assist you:
In the meantime you can directly run the PageSpeed tests from the built-in tool in Chrome:
It will provide you with the same exact data, as the PageSpeed website.