I don't understand why everyone keeps asking what retailers are excluded from the data as if that would lead to the "Scooby-doo moment".
If Walmart or Amazon is missing from the Transformers Nielsen data, then it would also be missing from the 300 Blu-ray data. The Nielsen data represents an apples to apples comparison and is probably the best we're going to get.
I can't see why the individual retailer market share for HDM would have shifted significantly for this release. Statistics are often based on large, sampled data.