Hi Hwang,
As Lorenz referred, the Lucene score is not a proper text similarity score, it's just a value used by lucene to decide if a result is relevant to a query or not.
If you need an actual similarity score, you can pass the results through a similarity metric, like the ones given by the kibbles-string-metric referred in this post. Just add the release jar to Stardog's classpath, restart the server, and several distance metrics will be available in SPARQL.
-pedro