Issue with evaluation context/context transition

Dear all,

I've got lost a bit in all types of filters and transitions of Power BI and would appretiate a lot your help!

In my dataset I have columns for Stock and Orders in items that should be delivered. The objective is to create a measure that will evaluate how many  items I can deliver, so the logic is the following: Match = IF (Stock>Orders) then Orders else Stock (example: I have 10 items of product A and orders for 15 items, so Match = 10 (statement IF (10>15) is FALSE).

This calculation should be done on row level, therefore I've created the following Base Measure to calcualte Orders and Stock:

Stock Qty = 
    SUMX( 
        Stock,
        Stock[Stock Qty] 
    )

Open Orders = 
    SUMX( 
        'Open Orders',
        'Open Orders'[Quantity] 
    )

As well as the formula for Match Measure:

Match (Stock) = 
VAR _NetStock = [Stock Qty] - [Open Orders]
VAR _Result =
    IF(
        _NetStock >= 0,
        [Open Orders],
        [Stock Qty]
    )
RETURN _Result

It works well on the lowest level of the granularity, but fails on aggregations. I expect 20 as the result for Herarchy Level 3, instead the result is 27.

Any ideas what do I miss?
Thank you very much!