Could this be caused by how they calculate the VAT amount?
(i.e. calculating a tax value for total qty of items instead of each item)
If an item has a 23% tax, the price is €9.76 before tax, or €12 (12.0048) with it.
For two items, that's €19.52 without tax, and after adding the 23% you get €24.01 (24.0096).