Tax rate on invoice is not consistent

Hello,

I use the Booster invoice module and I noticed that the tax rate of 21% in my case is not displayed correctly on the invoice. This is only the case with order ammounts of 1,00 euro or lower. The tax rate is 21%, but with low order ammounts the tax rate changes to 21,01 or 20,48% on the onvoice.

To be clear, it is not the amount tax charged but the used tax rate on the invoice.

Is this a bug or a setting?

Kind regards,
Sjors van Kruijsbergen