Part1 Annotating Exercises not understanding

Write code to get: •Customers with their last order ID

what I expected:

  queryset = Customer.objects.annotate(
        last_order_id = ('order__id').order_by('order__placed_at') 

which solution is

in database, the last order by user 1 is 321, the solution gives 931
321,2021-03-17 00:00:00.000000,F,1
403,2021-02-25 00:00:00.000000,P,1
540,2020-08-25 00:00:00.000000,F,1
544,2020-10-31 00:00:00.000000,F,1
931,2020-12-19 00:00:00.000000,C,1

why solution use MAX? why order__id not order_id

I find out my solution that satisfied me

    latest = Order.objects.filter(customer=OuterRef('pk')).order_by('-placed_at')

    queryset = Customer.objects.annotate(