My extension is in the no compression list, but file gets compressed

[cobian]

I have the extension *.xxx in the list of No compression, but te files get compressed anyway. Why?

You can't both have the cake and eat it. You indicated to us compression, so the files get compressed. No compression means, include the files in the archive BUT use a level 0 compression, i.e., just store these files without compressing them.

This has been the standard behavior of any archiver since the late 80s: zip, arj, rar, winzip, winrar, you name it.