I see the problem.
your zip is stored in / My zip i stored in /cassa
with your zip.aia i got a file cassa.zip containing the folder /cassa
So unzipping restores this folder cassa.
My cassa_a.zip containing the /cassa/a folder i stored as /cassa/cassa_a.zio
So it creates a folder /cassa/cassa_a/a/....
Now I copied the file to /tmp/cassa1.zip
doing your zip I got /tmp/cassa1/cassa/....
So your zip extension only gives the results when stored in /
in other directorys it makes absotule path to relative path. And normally one want to store zips in other folders.