It must be an oversight, right? Because why else would Black Widow not be included in this set especially since they had no problem adding Ultron. I don't see the point of omitting her.
Seeing this set advertised in the Sunday ad was disappointing because yet again Black Widow is missing from merchandising. Target perhaps you don't realize that Black Widow is indeed an Avenger which means that she should not be overlooked. Marvel (and Disney) should have already taken notice of this discrepancy since I'm sure someone signed some kind of contract for this product.