because the 6850 is re-branded of 5850.
you serious? It's not.
AMD changed the naming scheme after the HD5xxx series. Previously, the HDx8xx (Eg 4850, 4870, 5850, 5870 etc) was the high end series. After the HD6xxx series, the numbering changed, and AMD added another series, the HDx9xx series, and the x8xx series was the new high end mainstream segment, one which hadn't existed before.
Therefore, even if you are comparing a HD5850 and HD6850 and the seem to belong to the same graphics class, they dont. The HD6850 is a mainstram card and the HD5850 is an enthusiast card from the previous generation.