See for yourself
First Group's forum permissions - first you select one group, then you select any amount of forums and set the permissions for all of them (or for just one if you want).
On the other hand, when you use Forum permissions, the forums are primary. You first choose a forum, on the next page, you will see all the users and groups having some sort of permissions set for the forum, and you are able to change permissions for multiple groups.
Group's forum permissions - setting permissions for one group, more forums
Forum's permissions - setting permissions for one forum, more groups