Formula collision

I have created a formula in my query which is :

Per unit price = sales value / sales qty.

I create another formula which is :

Stock value = Stock qty * Per unit price

Now when I run the report at Brand level , this is what I see

Brand  Stock Value

ABC      11.09

Then when I drill down by Material , this is what I see

Brand  Material Stock Value

ABC      Mat1     10.8

         Mat2     .10

Total             11.09

If you see that 10.8 + .10 IS NOT 11.09.

I see that the issue is with forumula collision. The formula collision is kept to " Result from this formula".

I am not sure how to correct this. Pls advice

Thanks