Say that we have population A - this population gave rise to all human and all neanderthal populations.
Population A gave rise to population B and C and D and E and F. Populations B and C were closely related, interbred, and shared common ancestry with each other more than either does with D, E and F.
Population B leaves Africa and is the founding population that gave rise to Neanderthal.
Populations C, D, E, and F stay around Africa.
Later population C leaves Africa and is the founding population for all non-African humans. Population D, E and F stay in Africa.
Population C that gave rise to all non-African humans STARTED OUT more closely related to population B, and needed no interbreeding with population B to be more similar.
Population B dies out, leaving only populations C, D, E, and F - the founding populations of all modern human population groups.
The similarity of populations B and C could have come about because they started out more similar due to them sharing a more recent common ancestor. OR it could have come about because there was a bit of interbreeding between populations B and C. OR it could be a combination of both scenarios.
Good job with that...much better than my attempt.