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The reason for the behavior you are seeing is that the grand total lines are calculated separate from everything else. When you use a table calculation such as LOOKUP, you will often see that nothing appears in the total row because it is a single row. There are no rows before it or after it -- so nothing to lookup.at any offset, except 0.
They can solve almost any problem and we could even make it work in this case (see Customizing Grand Totals – Part 1 | Drawing with Numbers )
Also check out this thread: Totals for column using lookup function not showing.
Hope this helps.