I reckon your missing gigabytes are now in the Acronis Secure Zone, which contains your system backup. This is a protected part of your system which Windows can't access.
You can remove the Secure Zone (look under Tools on the L/H side of the Acronis opening screen and click on Manage Acronis Secure Zone). Removing the Zone will delete your backup, but if you're planning to start again from scratch this won't be a problem.
Hope this helps