I read that as:
You have <compensator rating> Worth of Stun and/or physical.
So: a rating 4 DC, would give you 0 stun 4 physical. OR 4 stun 0 physical OR 1 stun 3 physical. Or any other combination of damage up to the total of the compensator.
As to WHY it was changed... Keep in mind this is only my opinion, with no communication from anyone at CGL, nor a freelancer, and is expressed in my own words.
"To stop rules lawyering armor and DC stacking, cheese monkeys from being a the whiny, rules lawyering armor and DC stacking, cheese monkeys that they are?"
Do you KNOW how many times I heard the argument in 4e of: "Well, I have a Damage compensator of 9, and a stun track of 8. So OBVIOUSLY the damage compensator makes me totally ignore stun damage, including falling unconscious when it's exceeded. Combine that with 45 points of armor from loading armor into all my cyber limbs, demal plated, and welding 3 shields - 1 for each ass cheek and an other directly to my face, I am totally unkillable as nothing can penetrate my armor. THUS all attacks that don't penetrate have ZERO effect! Which for the record, is nothing short of a combined Thor shot."
And other types of Cheese lawyering that went on? As to why they are not ALL like that... different writers for each section? The cyber section just happened to have been a freelancer who got VERY sick of hearing the above argument ad-nausium??
But, that's just a guess...