I understand you have questions on how Revit establishes where the handrail support goes relative to the handrail? In this case all dimension for the Handrail Bracket are Managed in the Handrail bracket Family. You need to open that family in Edit Mode to make changes. However, like any Family in Revit I suggest saving it under a new name prior to making any changes. Then only make one change at a time, insuring that it works as expected in edit mode and in the project. Only move on to the next change when you are convinced that the last one works.
The attached screenshots should help and thanks for posting.
