manner of washing the arms

Salam Alaikum,

I recently came across a hadeeth in the Amali of Ahmad bin Isa bin Zayd AS stating that Imam Jafar Al Sadiq washed his arms from the elbow to the hand, could you shed some light into this?

ينارست :علاق دمحم نب رفعجل عناك ةيراج نع ،بلاط يبأ نب يلع دلو نم يش ينلدح :لاق ،دمحم انلدحو :لاق ببو ،بيلع يصحأ عنكو :علاق ،يلع يصحا :لوقيو ،اللال اللال أضوتي ناكو ارفعج يضوأ عنك :علاق ،عدلوو دمحم بنبا عأض و ت ي ل لا ق ام بر ف لوقيو ،بيقفرم نم نهرخآ أدبي ناكو ،ىرخأ لسةيف :علاق ،نيتنلتأضوت ،معن :لوقأف ؟نيتنل فكلا ىلإ قفرملا نم ينعي ،انه اه نم :نهرخآ.

A shaykh from the children of ʿAlī b. Abi Ṭālib narrated to me from a slave girl who was Jaʿfar b. Muḥammad’s. She said: His son Muḥammad took me as a concubine slave and I gave birth. She said: I used to assist Jaʿfar in the wuḍū and he would do wuḍū thrice, and say: Count for me. She said: And I would count for him. So sometimes he would say to me: I did wuḍū twice? So I would say: Yes, you did wuḍū twice. She said: So he would wash another (time), and he would begin their end from his elbows. And he would say: Their end: From over here, meaning from the elbow to the hand.