When reviewing the Top 15 or 20 Projected Cost by Group, AWS Savings Plans or Reservations may not be represented.
Due to the behaviour of Unused Savings Plans and Reservations and how they 'burn-down' / are apportioned as a month progresses. Unless a there is consistent unused cost at the end of a month, Unused cost will not be included in the projections as by the end of a month there is no cost left in the 'Savings Plan - Unused' to be included in the projection. These costs are apportioned into the consuming service (EC2, etc.)
For further reference on burn-down, please see Savings Plans - Unused costs in the Cost History reporting