eBay says that the DSR scores left by the buyer are anonymous. So if the DSRs left by a buyer immediately showed up in a seller's overall ratings, it wouldn't be anonymous, since the seller could tell what scores were left, if he/she was keeping track.
I'm about 90% convinced that there is a time lag of several days, maybe even a week, before ratings left are accurately reflected in the overall scores. The time period may be somewhat random, just to keep everything anonymous.
That's the best sense I can make of it, since some very positive feedback by buyers has been followed by DSRs going down, only to "level off" and be where one would expect them to be, after awhile.