Buy get deals in RMS is also giving percentage off on buy item

Hi,

Create a Buy get deal

buy X of 20 items and get Y of 10 items at 90 % discounted cost

before applying deal

X item cost 100 and qty is 20=2000

Y item cost 100 and qty is 10=1000

total =3000

after applying deal

X item cost 95 and qty is 20= 1900

Y Item cost 10 qty is 10= 100

Total PO cost 2000

Now the calculation of the deal is

(X of 2000-Y of 100)=1900

1900/total X item qty 20=95

it becomes 95 has unit cost of x item

but the deal applied on Y item and it is prorating the deal on both items and making the PO cost to 2000 which is wrong according to my understanding,