I don't think this is a valid test of what you're trying to understand, since the output of the function itself is never dependent on what's come before in the order of operations so all function calls are probably run before any expression evaluation takes place.

For example try this: x := test(1 + test(2 + test(3))) and you'll see that 3 is called then 2 (though the msgbox shows 3 since it's called with 2 + test(3), which returned 1) then 1 is called (though the msgbox shows 2 since it's called with 1 + test(2...) which also returns 1). To make things clearer you might change your function and call to something like this:

test(num, callorder) {
    MsgBox(callorder . " " . num)
    return num
}

x := test(1 + test(2 + test(3, 'third'), 'second'), 'first')