I suspect it is based on how they structured their Air Force, similar to the RAF - a Navigator could be badged Nav but actually operate as a helicopter crewman.
The RAF put Navs in the LHS of Chinooks for a while - they were doing most of what a co-pilot would do - comms, nav, flight planning, fuel management etc.
Most Navs on helicopters came across from FW - as officers rather than the mostly NCO crewmen - and performed many of the same functions.