Comment 12 for bug 1108801

the problem is that it is perfectly valid to have two different taxes at different rates contributing to the same base code and tax code for the tax report. If you group taxes from invoice lines based on them having the same base and tax codes then you get the wrong rate being applied.
Zero rated is probably not the best example as that doesn't need a tax code so could be different to the standard rate one, but consider an invoice with some lines at lower rate vat at 5% and some at standard rate vat at 10%. It doesn't make sense to collapse two differently named taxes onto one line just because they contribute to the same totals on the tax report.