I can think of a few reasons she may want a trial at a lower dose.
1) the long-term effects of stimulant use have never really been studied and adderall’s own medication guide suggests trials off the medication when used long term to make sure it is still needed and to assess the dose needed
2) in general doses higher than 40mg total per day are rarely more effective (not saying that is the case for you)
3) when someone needs that high of a dose of short-acting stimulant its always a good idea to try a long-acting form. If it works you get the added bonus of only needing to take it one time per day, and if not you can always go back to your current medications
4) stimulants, like every other medication, have the potential for side effects. Given stimulants can affect the heart I would hope the APRN just has your best interests in mind