Services leaving Cannon Street (to return to London via Bexleyheath, Sidcup or Greenwich) will always be advertised as terminating at Slade Green until they reach Eltham/Sidcup/Woolwich Arsenal.
Then, depending on what route the train is taking on its’ way back to Central London it will market itself as terminating at Eltham/Sidcup/Woolwich Arsenal.
Once it reaches Slade Green, it will advertise itself as terminating at Cannon Street.
It does make sense for some of the reasons already pointed out. Additionally, passengers from stations like Lewisham that want to travel to stations like Woolwich Arsenal would probably be misled by a service terminating at Woolwich and a service that goes to Dartford via Woolwich.
The loop service is honestly great, my only issue with it is unnecessary padding at some locations, and also train drivers sometimes “forget” to change the destinations at the points above. This is very confusing for passengers that are not familiar with the system, especially if you aren’t starting your journey from the London terminals (the only location on most of these trips where they will bother to make announcements to correct the record). I think this tends to happen most when drivers changeover at Slade Green, but that’s just a theory. I’m hopeful that on the new stock they will have this happen automatically so it’s not down to drivers to fix the destination.