Big difference between geant4 and mcnp6

Geant4 and Mcnp6 are used to simulate 0.565MeV neutron to bomb B-10 target with thickness of 2e-6 meters, Big difference occurs between geant4 and mcnp6.

here is result from Mcnp6:

here is result from Geant4:

the foece collision of variance reduction tecnique are used in both programs.
it seems the result from Mcnp6 is more correct.
what special shauld i do in Geant4?