I think (and could be wrong), its because there's a big fat rendered data file in there, not a Raw (or might be Raw + rendered file).
A DNG isn't necessarily containing a non rendered, non demosic'ed data.
I am still finding the whole concept a little confusing, but this is what I think I understand:
> If LR takes a CR2 (or other) RAW file, and converts it to DNG, then it is still a RAW file, but in a different format. LR does the RAW conversion on the fly, as you view and work with the DNG images.
> If some other program (such as DxO) takes a RAW file, and converts it to DNG, it may no longer be a RAW file, but rather it is already converted. Somehow LR recognizes it, but is no longer acting as a RAW converter.
Is that right?