No worries.
Of course your logic makes sense, and it could be the same master indeed. But I'd absolutely believe if an insider would tell us that the DVD master was too low quality and they've done a higher resolution scan from the same print for the Blu-ray. I simply couldn't find that out just by looking at the releases. I can only see they had a common source at one point in time (hence the similar dirts), but that's it.
But after all, it's not even important. It doesn't help us.