If I understand correctly, by adding the exception of http://www.sisprima.com/product.html, you have told avast that it should block all of the pages related to http://www.sisprima.com (indicated by the *) except the page added in the exceptions.
The webshield exceptions function to allow you to exclude websites that may be identified as infected from the scanning. This may be due to false positives etc. (care is needed here)
If you want to add exclusions for other scanning then you will have to ad it them in another section.
Now that I think about it…
Would adding the slash mean that the home page (www.sisprima.com) is not included in this exclusion, only the pages with extensions to the site address?
After i observe it and read the help information for exceptions feature only for exclude the files which allowed to download from URL.
So that function was created not for URL visited exceptions, but only for downloaded file exceptions.
Hopefully in the next versions, it would be include for URL exceptions feature.