Gary-

I'm a little bit puzzled why the radiation level was 2.5 times greater than the levels experienced at ISS. I think(?) this greater value maybe due to back scatter of secondary cosmic rays (GCRs) from the Martian regolith. The Moon should have a similar radiation level if this theory is correct.

I'd say that the fact neutron and gamma
ray spectrometry has been used to map the surface of both validates your point.

Larry-

The van Allen belts absorb quite a bit of the solar radiation. Flights beyond the belts have to allow for higher radiation levels.

The magnetic field of the Earth (of which the VA "belts" are a direct result ) also deflects some high energy particles as well as traping or scattering them into belts. During solar particle activity (SPE, CME) the belts can become quite a hazard as they trap large populations of high energy particles (mostly protons and electrons).

Also as I'm sure you well know from Oberg and others, the ISS traverses the lower portions of the VAB closer to the poles thanks to it's Baikonur accomodating orbital inclination of ~51 deg.