aapt -u flag does not seem to have any effect

gistfile1.txt

> time $ANDROID_HOME/build-tools/android-4.4/aapt p -v -M AndroidManifest.xml \
     -S res/ \
     -F resources.ap_ \
     -I $ANDROID_HOME/platforms/android-19/android.jar

...
real	0m15.602s
user	0m49.074s
sys	0m1.695s

> touch res/drawable/logo.png
> time $ANDROID_HOME/build-tools/android-4.4/aapt p -u -v -M AndroidManifest.xml \
     -S res/ \
     -F resources.ap_ \
     -I $ANDROID_HOME/platforms/android-19/android.jar
Configurations:
...
Opening 'resources.ap_'
Writing all files...
      'res/color/attr_in_color.xml' (compressed 47%)
      'res/drawable/attr_in_drawable.xml' (compressed 53%)
      'res/drawable/logo.png' (not compressed)
      'res/drawable/logo1.png' (not compressed)
      'res/drawable/logo10.png' (not compressed)
      'res/drawable/logo100.png' (not compressed)
      'res/drawable/logo101.png' (not compressed)
...
      'resources.arsc' (not compressed)
      'res/drawable-hdpi/email.png' (not compressed)
      'res/drawable-hdpi/facebook.png' (not compressed)
      'res/drawable-hdpi/gplus.png' (not compressed)
      'res/drawable-hdpi/ic_launcher.png' (not compressed)
      'res/drawable-hdpi/larry.png' (not compressed)
      'res/drawable-mdpi/ic_launcher.png' (not compressed)
      'res/drawable-xhdpi/ic_launcher.png' (not compressed)
      'res/layout-land/qualifiers.xml' (compressed 49%)
Generated 536 files
Included 0 files from jar/zip files.
Checking for deleted files
...
      (removing crufty 'res/drawable-hdpi/ic_launcher.png')
      (removing crufty 'res/drawable-hdpi/larry.png')
      (removing crufty 'res/drawable-mdpi/ic_launcher.png')
      (removing crufty 'res/drawable-xhdpi/ic_launcher.png')
      (removing crufty 'res/layout-land/qualifiers.xml')
Removed 536 files
Done!

real	0m14.437s
user	0m49.225s
sys	0m1.497s