I don't understand how you get that.
Here are my results on the demo website:
That is without calculations rounding.
The price of the product is €8.3612
When you multiply by 10, it's €83.612
Then you add the taxes (19.6%) on it: 99.999952 which is then rounded to €100
The taxes are: 99.999952-83.612=16.387952 which is then rounded to €16.39
So these calculations are perfectly correct.
That is with the calculations rounding.
The price of the product is €8.3612
When you multiply by 10, it's €83.612 rounded to €83.61
Then you add the taxes (19.6%) on it: 99.99756 which is then rounded to €100
The taxes are: 100-10*8.36=16.40
So these calculations are perfectly correct too.