The only thing I can think of is MVA is generally associated with having multiple adapters between a view and model. While there's nothing stopping you from doing that in MVP, I think MVP implies a strong triad.
Update (years later): in retrospect, I believe a key difference would be control flow. In MVP, the View triggers/creates/calls the Presenter, which delegates to the model and responds back to the View. In MVA, when messages come in, an Adapter is chosen, which then mediates between the Model and View. Because the MVP View triggers/creates/calls the Presenter, choosing between multiple Presenters (as an Adapter is chosen) is not a straightforward fit.