I stumbled upon this today from the BFI:
As Senior Curator (Fiction) at the BFI National Archive, I'd like answer the points raised over our work on DRACULA. The work undertaken by Warner Bros in the mid-1990s was not a restoration as such but simply the preparation of digital materials for a DVD release. The BFI has prepared new preservation materials on film from the original negative. The new version, incorporating the original UK title sequence, benefits from additional technical work that has been carried out on both picture and sound. Furthermore, we have reinstated a brief sequence which was cut from the UK release version by the BBFC.
So that clarifies the mystery a bit and a big crux of the statement is as follows, "The BFI has prepared new preservation materials on film from the original negative." When you make a scan of the negative it's not truly presentable for either film prints OR media releases. Not only because it is literally a negative (all the colors are reversed) but there are many tweaks that need to be done to make a version suitable for release as well. These steps are all different for preservation material, theatrical prints and media transfers.
So any scan of the negatives would need further adjustments for Blu-ray besides what the BFI did for preservation/theatrical release. Also Hammer is adding back in additional cut footage found in Japan which was acquired after the BFI restoration. That specific footage is in need of desperate need of restoration as well. So that's why they are continuing the "restoration."