Maybe they think the accident scenario of running into a huge block of concrete on the highway is uncommon? I'd look at the "real world" insurance claims results, also available from the same source.

Compared with the 850/70 it looks like the s60 recieved one additional "Acceptable" rating.

http://www.hwysafety.org/vehicle_ratings/ce/html/95005.htm

There have always been safer cars than Volvos, and the testing methods have always been less than perfect. In one of the very first crash tests note the 1979 240 received one (1) star, same as the Renault LeCar.

http://www.crashtest.com/explanations/archive/crash.htm#anchor161663

-Punxsutawney Phil