Tickets and reservations are two completely separate entities. With electronic tickets, the ticket is "hooked" to the reservation. Changing one does not necessarily change the other. If you change the PNR, the ticket will not automatically update, and will subsequently contain different information until it is changed (hence where "reissuing" comes into play).

Electronic tickets function remarkably similar to paper tickets and understanding them is much easier if you think of them similarly. If you think of having physical "coupons" for your flights (paper ones, like you used to get in the mail or at the airport when you bought a ticket), calling the airline and saying you want to take a different flight will update your reservation, but you still have a paper coupon that says you are ticketed on your old flight. Hence, the coupon needs to be "exchanged" for a new one that matches your flight. An electronic ticket works no differently, the only thing that makes it different is that it is happening on a computer instead of you being issued paper coupons.

I hope this helps!

~WK