It probably relates to the type of lease agreement entered into.
Wet or Dry Lease.
Wet lease - tends to be with the crew etc and the aircraft will operate under the banner of the airline it has been leased from.
Dry lease - only the aircraft is leased, with all crews etc supplied by the airline using this aircraft. As they operate the aircraft with their own crews the aircraft will be
re-registered to that countries requirements.
Hope this helps :-)