The GRV98lo PDF that comes with LHAPDF 5.9.1 has a small problem.
This file, provided by LHAPDF collaborator Graeme Watt matches
the implementation of GRV98lo as it appears in the old CERNLIB
PDFLIB.

Renaming this file to `GRV98lo.LHgrid` and overwriting the default
file for LHAPDF 5.9.x will "fix" the PDF problem associated with
GRV98.