What is the CEP for unguided artillery rockets? Say for MLRS, BM-21, Uragan, Smerch...?

Quick Google-Fu gives 9K58 Smersh MLRS maximum error of 220 meters and stated circular-error probable [CEP] of 120-150 meters at maximum range.

http://forum.keypublishing.co.uk/showpost....mp;postcount=39

Following: http://www.aviationweek.com/aw/generic/sto...0and%20Mobility

says:

The TCS is also claimed to increase accuracy to 50 meters CEP, a major improvement over the average of 100 meters CEP for artillery rockets like the Lockheed Martin M26, Russian GRAD (122 mm.) or the LAR (160 mm.) from IMI.

What do they mean by 'maximum error'? If CEP is 150 metres (radius) then 100% CEP is 600 m radius. I seriously doubt that it is possible to reduce the CEP of an unguided rocket at max range, eg 20km or more, to anything like 50m. Of course there's also the matter of whether CEP is a valid measure for rockets (it isn't for guns), even allowing for the effects of cross-wind at launch.