It is to do with the granularity of the queries that are run.
SQL aggregation will take place during the running of the query, Sum, Max etc, have you defined these for your objects?
Projected aggregation takes place on the face of the report using the Webi aggregation engine.
More info here:-
https://bobj-board.org/t/71712
Mak 1 
(BOB member since 2005-01-06)