Aft Grand Staircase Questions

So I know the main grand staircase was located on the boat deck and went to E deck, but why wasn't the aft grand staircase on the boat deck too? Was it lower or something?

[Moderator's note: There is a separate topic on the subject of the Grand Staircases, including at least one discussion of the aft grand staircase. This question should be addressed there; this thread is therefore closed. Use the Search function to locate the appropriate thread. MAB]