Koreen here from Shopify. Thanks for posting.
Apologies for any confusion with this. While we do aim to keep the system updated regularly here, tax rates can change quite frequently on us.
In this case, you may like to manually edit and set up your taxes. We have a guide here that can help you out. This will allow you to ensure you are charging the correct tax, based on your own exact location.
I've also sent you an email, just to get the details of the inaccurate rates being calculated, so we can look into a fix.
Hope this helps!
Koreen W | Shopify