Modifier Showing wrong in Sales order Screen

 HI ,

I HAVE A scenario THAT IS 

      ITEM 'A' HAVE MODIFIER LIKE  TWO LINES , DISCOUNT AND PROMOTION(FREE)

IN DISCOUNT WHEN APPLY MEANS 1-19 NOS

THEN PROMOTION WHEN APPLY MEANS AFTER 20 QUANTITY EACH 5 NOS 1 ITEM FREE.

scenario IS 

IN SALES ORDER SCREEN I PUT 60 NOS(QUANTITY) IN 1ST LINE THEN THE MODIFER WORKS FOR THAT ITEM AND SHOWS 12 NOS IN 2ND LINE

NEXT  I PUT SAME ITEM 40 NOS(QUANTITY) IN LINE 3rd  NOW THE MODIFIER WORKS FOR 100 NOS AND SHOWING FREE ITEM 20 ON 4TH LINE.

TOTALLY FREE ITEM 34 NOS INSTEAD OF 20.

THIS IS WRONG PLEASE ANY ONE HELP ME...