The Delta rocket started out with separate versions of the aft end for different numbers solid rocket boosters attached. Eventually McDonnell Douglas standardized, introducing the Universal Boattail Thor (UBT) with with a full complement of nine attach points regardless of how many solid boosters were to be flown on a mission. The positions for the boosters were not evenly spaced on the UBT, though unlike for some Atlas V configurations the aggregate thrust was always symmetrical. 

As I'm sure was the case with Atlas V, MDAC's decision to use the same aft end regardless of the number of solid boosters flown on a mission was founded in engineering analysis which traded performance increments against program cost increments, and took consideration of reliability intangibles such as that the more versions of something you fly, the more chance for problems and the less flights each version gets compared to if there is just a single one... diminishing the demonstrated reliability for any of the various configurations relative to only a single type.

Finally, it appears from inspection (i.e. I don't specifically know, but I do have a structural engineering background) that these "attach fittings" are structural elements which carry loads for the frustum.  That would allow the frustum and associated internal structure to be made lighter, so the attach fittings would thus not be deadweight even if no booster is present at a given position.  More like that they are external "ribs", which then do double-duty when there is a booster attached.