They might be the same class of objects that differ only in the direction in which their radio-emitting lobes are oriented compared to our line of sight.
Core-dominated galaxies might be lobe radio galaxies that have exhausted their central material and thus no longer produce lobes when viewed in radio wavelengths.
Lobe radio galaxies might be core-dominated objects in which our view of the core is partially obscured by intervening gas and dust.
They might be black holes that differ from each other only in whether or not they have formed an accretion disk.
They might represent different stages in the evolution of active galaxies and quasars.