Hi David,
To my understanding, it is working perfectly and there is nothing wrong in the behaviour.
Consider TABL_1 as header table with Order and Value. TABL_2 as item level table with Order, Line and Value. If value of an order is 200 and it has 5 lines with values 40 each, when we query at header level value should be 200 at any point of time, but not 1000(200 * 5) after joining to line item and removing line from analysis.
Similarly, even here the fields selected MY_ID and MY_NUM are from the same table. Hence the result is 2 but not 4 at any instance.
Hope this clears your doubt.
Regards,
Chandra.