Predicting line item tax rounding

[PublicApps]

When

> I create a draft order with a single line item having €1,004 tax:
  the line item tax is rounded down to €1,00 and total invoice tax is also €1,00. (OK)
> I then add a second line item with €1,004 tax,
  this second line item's tax is also rounded to €1,00, but the PREVIOUS line item's tax is suddenly ROUNDED UP to €1,01 tax to make the total invoice tax equal to €2,01 (which is €2,008 correctly rounded as far as I am concerned).

So how can we predict which line items get adjusted up/down on the invoice to? How does the software decide this?