I just don't understand why it would specify ticket number otherwise
All e-ticketing, air or Amtrak, has ticket numbers as well as a reservation number/PNR. The ticket is the entity which actually carries the value. The reservation number/PNR ("Passenger Name Record") is a reference that enables easy reference and carries additional passenger information. They are linked, but they are separate.
Think of e-tickets in terms of old paper "value" tickets. They were called "value" because the ticket is a proxy for the monetary value. With paper tickets, you had a reservation number/PNR but you had physical tickets, too. You could not board a train or airplane with just evidence of a PNR, you had to present a ticket and have it collected. On airlines, you were usually given a boarding pass in exchange that allowed you onboard, on trains, the conductor just took it and pouched it, turning the tickets into accounting at the end of his run. With e-tickets, there is still a ticket there, separate from the PNR, just as physical tickets were, but instead of carrying it, it is held for you electronically and "collected" when scanned. The big advantage is you cannot lose an e-ticket like you could with a paper "value" ticket.
E-tickets are usually so seamless that for practical travel purposes, the reservation number/PNR, the e-ticket and the boarding pass feel like they are all one thing. They really aren't, but feel that way until there is a glitch with e-ticket issuance. Then the difference between an e-ticket and a PNR becomes painfully clear.
Since you have a ticket number, you are not in that position, so have nothing to worry about and need not be concerned with the backend artifact that is the actual e-ticket.