That seems to tie in with the operation of our 737s. The old ones are plain CAT IIIa and fail passive, limit is 200m RVR and 50ft DH. On the newer ones however you have fail active operation with CAT IIIB at 75m RVR and no DH, however if those are in LAND2 you still can get an approval for a reduced RVR limit of 125m RVR albeit with the usual 50ft DH.