Where is the tax coming from?

By default, the dummy variants generated by the app are taxable. The app doesn't change or add any tax in addition to your current setup. 

If you've set up taxes in your Shopify admin => Setting => Taxes, there will be a tax added to the checkout. 

From the screenshot, there is a tax of $3.3. Therefore, there is 3.3/55 = 6% of tax in your tax setting.