Hi,
First of all, are you sure that Google didn't take the migration into account?I just did a quick check on other https sites. Example: when I look for "Google Analytics" in Google - the first 3 results are all pointing to Google Analytics site, however only for the 3rd result the https is shown, even when all three are in https. So it's possible it is just a display issue rather than a real issue.
Second, I did a quick crawl of your site and I noticed that on some pages you still have links to the http version of your site (they are redirected but it's better to keep your internal links clean - without redirections).
When I checked one of these pages (https://www.gomme-auto.it/pneumatici/pneumatici-cinesi) I noticed that this page has some issues as it seems to load elements which are not in https - possible there are others as well.
example: /pneumatici/pneumatici-cinesi:1395 Mixed Content: The page at 'https://www.gomme-auto.it/pneumatici/pneumatici-cinesi' was loaded over HTTPS, but requested an insecure image 'http://www.gomme-auto.it/i/pneumatici-cinesi.jpg'. This content should also be served over HTTPS.
The page you mention as example: the http version still receives two internal links from https://www.gomme-auto.it/blog/pneumatici-barum-gli-economici-che-assicurano-ottime-prestazioni and https://www.gomme-auto.it/pneumatici/continental with anchor texts 'pneumatici Barmum' & 'Barum'
Guess google reasons, if the owner of the site is not updating his internal links, I'm not going to update my index
On all your pages there is a part of the source which contains calls to the http version - it's inside a script so not sure if it's really important, but you could try to change it to https as well
My advice would be to crawl your site with Screaming Frog, and check where links exist to http versions and update these links to https (or use relative links - which is adviced by Google (https://support.google.com/webmasters/answer/6073543?hl=en see part 'common pitfalls')
rgds
Dirk