Navien CMB-H manifold kit dielectric union

[Mark Burbank]

I purchased a Navien NCB-H combi-boiler manifold kit (P/N 30026576B). The manifold itself is fabricated from stainless steel so it's designed to be attached to the boiler and the building plumbing via dielectric unions. The union nuts are bronze and the kit includes rubber isolation gaskets for all the connections. However, the two 1-1/2" tail pieces (MIP/FIP) for the hot water return are stainless steel. Does anyone understand why the tail pieces are stainless steel instead of bronze?