That would be a country specific thing & is a bit of an outlier. As a simplified example though - sometimes the sales tax / VAT is calculated as 10/110 instead of 10/100 (or vice versa depending on law). Or there is some small piece of info missing that technically means the invoice isn't a legal tax invoice which means whoever you issued it to claims tax on that illegally (again country specific).
It's unfortunately a case of people dont know how much they don't know.
Can you say more about this?