Why Shia and Sunnis Salat is different

This question is stuck in my head from quite some time now that why shia and sunnis are offering namaz in a different way, in fact, most of the namaz part is different. I am a Sunni Muslim and one of my friends is Shia Muslim her namaz is quite different from us why it is? We all are Muslims and we should pray in the same way. **Which way is authenticated and which one is recommended by Rasool(SAW).** I searched a lot and most of the places I found(our namaz method).