I tried to make a differents tests compression with 7-zip and this it' amazing!!
After the several tests in the SAME session i compressed a 1 gig of data.
And i rebooted the machine (too slow after too many compressions with ultra and 32mb of dictionary)
And i i don't know why (maybe an error) i packed again the SAME directory and now it's 100kb's less!!!
How it's possible? Depends of the use of the memory?
Scommetto che sei italiano! ;)
(I bet you're Italian)