Random sales tax with coupon code

[artypants]

Hi, I've just had an across the board 25% off sale via coupon code. The tax on the theorders in which a coupon code was used was incorrect. The drgree of error was random: in some cases it was slightly off and others it was way off, but in all cases they were undercharged for their sales tax.

 

Any ideas on this?

 

Thank you.