Why does Fast Scala Compiler compile when not needed?

I have been using IDEA X and the Scala plugin is much better. Autocompletion is greately improved and overall I'm really happy.

However I still don't get what I do wrong with FSC. When I run something (Ctrl+Shift+F10 or Shift+F10) I get the followig:

• Without FSC and no file has changed execution is immediate
• Without FSC and some file changes, they get compiled (something like 90 secs go by).
• With FSC even if not files have changed a sub-set of the files are still compiled (around 80 sec)

FSC is blasing fast for a single file, and a bit faster than the regular make for a small set of files. However with the need to recompile files that have not changed, the bonus in speed is not so great. I also thing that as more and more files get modified the time increases.

I was wondering if there is something I should configure to make FSC aware that no files have changed?

This is my FSC command line (I run on Windows XP):

"C:\Program Files\Java\jdk1.6.0_22\bin\java" -Xmx1024m -Denv.classpath="%CLASSPATH%" -Denv.emacs="%EMACS%" -Didea.launcher.port=7532 "-Didea.launcher.bin.path=C:\Program Files\JetBrains\IntelliJ IDEA 10.0\bin" -Dfile.encoding=UTF-8 -classpath "C:\Documents and Settings\Administrator\.m2\repository\org\scala-lang\scala-compiler\2.8.1\scala-compiler-2.8.1.jar;C:\Documents and Settings\Administrator\.m2\repository\org\scala-lang\scala-library\2.8.1\scala-library-2.8.1.jar;C:\Program Files\JetBrains\IntelliJ IDEA 10.0\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain scala.tools.nsc.CompileServer