The cause is the fact that they have orbits of different size.
For two objects in orbit around the same central body, the one in the
larger orbit will take longer for each complete revolution.
And if two objects are in the same-size orbit, their periods of revolution
are identical, even if the objects have different size and mass.
(Example: All TV satellites are in the same orbit. Some are big and heavy,
some are smaller and lighter, but all of them revolve around the Earth in
exactly 24 hours.)
