why the tax code in main/sub for consumption is picked by the line items during BBP creation in dfkkmop

Hi Experts,

I have an issue : please refer to the thread http://scn.sap.com/message/14860046. where the tax determination code mentioned for main/sub 100/202,203 is picked by the line item of BBP in DFKKMOP. This is strange for me because main 100 is for consumption billing and 051 for budget billing, then why the TAX determination code in main 100 picked for BBP.