Being assigned 20% of blame for Rear Ender

I was rear ended. The vehicle that hit me was enterprise rental.

I claimed from Enterprise. Today they informed me they are taking 80% liability.

How can that be? I thought rear enders are usually 100% fault of driver that hits the car in front.

What should I do? Should I argue with them for remaining 20% before accepting? Should I accept and then argue over 20% remainder? Should I talk and file with my insurance coverage and let them try to get my deductible back?

Anybody who dealt with this kind of situation, please advise?