CLucene aimed for binary compatibility with Lucene indexes, and as a result had very little room to innovate. Lucy started out with the same approach, but decided early on that it was better to take the best parts of Lucene's internals while not being bound to all of them.

Some parts will be leading Lucy, and some will be catching up. There's already increasing cross-pollination between the two. It's a very loose port at this point.