Question about Alitalia's daily flight to JFK from Milan

I have been confused about Alitalia's daily flight to JFK from Milan. The flight is designated as AZ604 in the Alitalia timetable and on arrival information yet when researching for past AZ 604 arrivals, there is NO such flight designation listed. For reasons I can't figure, this same flight is designated as AZA 71A. What explanation is there for this discrepancy? It's not a codeshare number, either. I've never come across a regularly scheduled flight having 2 completely identifications. It's a passenger, not a cargo flight. I am really puzzled.