SBT compiles source files regardless of timestamp

I noticed that SBT will compile all the source files every time I issue the "compile" task, regardless of the timestamps since the last compilation. Below is my build.sbt file:

name := "HelloSbt"

scalaVersion := "2.8.2"

unmanagedSourceDirectories in Compile := List(file("src"))

The following is my project structure (project and target directories are ignored):

./src
./src/Hello1.scala
./src/a
./src/a/Hello2.scala
./build.sbt

The two source files are simply definitions of empty objects for testing purposes.

When I print "sbt compile", I got the following information:

[info] Compiling 2 Scala sources to...

and I can find the class files just compiled in the destination directory.

- , , "sbt compile" , , , . , , .

Windows 7, Debian. . - , src, ?