"Decline and Fall of the Sassanian Empire" by Pourshariat Parvaneh, defend that even during the Sassanian period, the Parthian families were largely influential and supplied the Sassanid armies with men and even generals. The Sassanians needed to keep a delicate balance between them to keep their power. Quite possibly the same happened to the Arsacid. So maybe the Parthian and the Sassanid armies are not that different?