LevelDB is based on concepts from Google's BigTable database system. The tablet implementation for the BigTable system was developed starting in about 2004, and is based on a different Google internal code base than the LevelDB code.[1]

[1] http://en.wikipedia.org/wiki/LevelDB#History

They probably release old tech after they have upgraded their own. So when they did their internal LevelDB 3.0 they released 1.0 as opensource. Probably do the same thing with all their releases (map-reduce,bigtable etc).