Help understanding SUMX / Calculatetable code logic

Hi,

I joined a new BI team recently and found the following measure:

Order Value =
SUMX (
     CALCULATETABLE ( Orders, Orders[Order Index] = 1 ),
     CALCULATE (
        SUM ( 'Order Items'[USD Value] ),
        ALLEXCEPT ( Orders, Orders[Primary Order] ),
        Orders[Order Index] = 2 ) )

What I don't understand is how this measure returns any value at all.

The inner calculate returns a sum filtered on  'Orders'[Order Index] = 2.

then there's a calculatetable relating to that sum, filtering the result to only 'Orders'[Order Index] = 1.

To my understanding, this should always return blank as you will not have any [Order Index] = 1 in a result alreay filtered to 2.

Is it made possible because of the "allexcept (orders...)"?

Thank you!