KLM 747 FLIGHTS

Why do KLM flights that use 747’s usually show as two separate flight numbers at the same time? For example, KLM 25 is always accompanied by KLM 661 while viewing tracking, however, there is only one plane in the air. And sometimes the phantom flight, which is 661, may not be anywhere near the flight path of the actual flight KLM 25. I see this only on 747’s.

Anyone? Bueller?

It might be callsign vs flight number. They are not always the same. For example, Lufthansa:

Munich to Chicago
Flight: DLH434, Callsign: DLH8P

and

Chicago to Munich
Flight: DLH435, Callsign: DLH5Y

Marty