Yup that's exactly it - 6 figure grid refs are only accurate to 100m. Some of the sites have been updated with better refs where the mapping is of sufficient quality to accurately get a long/lat for it, but there's a lot of sites in the database and the majority haven't been updated, so most will only be accurate to within 100m. If any intrepid souls feel like supplying more accurate grid-refs for things, then let us know and we'll set up some kind of submission form for you.
Thanks
HMCG