There are tobaccos that must sit for a while after opening, after which time they will improve.
There are tobaccos that are best when first opened, after which time they begin a gradual decline. Like a marriage the initial exotic charms begin to sublimate until you are left with the true structure of what lies beneath. If the leaf is quality material your tobacco will have evolved into something, softened and tempered, but pleasant and interesting. If the underlying leaf is monotone, little more than an insipid vehicle for the flavorings imparted upon it, then you'll be left primarily with a subtle suggestion, a remembrance of the glory of the newly-opened tin.
I've discovered that many of my opened tins don't have anything approaching the "oomph" that they had when I opened them. Plenty of the Robert McConnell flakes and ready-rubbed Virginias are all starting to taste the same.