Formula for calculating Tax on AR Invoice

Can someone please explain the formula for calculating Tax on AR Invoice(with several items + freight) in SAP Business One 2007.

These are the settings:

Administration > System initialization > Document settings > General tab. Rounding method is set u2018By Currencyu2019.

Administration > Setup > Financials > Currencies. Rounding is set to u2018No Roundingu2019 and Decimals is set to 'default'.

Administration > System Initialization > General settings > Display tab. Decimal Places:

Amounts 2

Prices 2

Rates 4

Quantities 3

Percent 2

Units 2

It seems that it calcualtes tax for each row(does not round to 2 decimals places)>sums tax for each row>rounds the total tax to 2 decimal places.

Sometimes it rounds like this ...10.498 to 10.50. Other times it rounds like this 12.9845 to 12.98(?). Can someone please explain the rules for rounding in SAP.