The reason it’s not calculating the way you’d expect is because the 10% GST on the ticket was actually set as a price modifier, which is not the same thing as tax.
Here’s what you do to set up a taxable amount:
1) Go to Event Espresso → Pricing and if no tax items are listed there, Add a new default price
2) Edit the tax so the amount is 10, save
3) Go to edit the event, remove the price modifier, and check the box next to “This ticket is taxable”.