I guess that's because the Weston was built with the Varne Moulds! The coachroof was raised a little to give 4" more headroom. They also played about with various keel designs - as did Varne before, but also offered lifting keel and twin keel options. Mojo was built with an encapsulated lead keel to allow her a further three feet of mast, just to give her the edge over her sister ship "Lotus Eater" a lifting keel.
To bring the comments back on topic, I believe all the Varnes were built with bolt on keels, either iron or lead and various depths and profiles seem to have been tried. I have never heard of anyone damaging the hull through grounding and many of them are sailed around their home territory around Maldon. I bought Mojo in West Mersea and being my first trip in the area, I can assure you all that I now have personal tactile experience of the sand in the Blackwater and Thames Estuary. I haven't, thankfully, found any sign of damage following these depth challenged moments!
Previously I sailed on a friend's T24 and the iron bulbed keel was abraded by the half tide mooring, so never had antifouling for more than a week or so. On the other hand, rust couldn't stay long either. We once tried to extract a keel bolt using all the available space to swing a sledgehammer onto a socket bar. Came out looking fine and went back the same way. I do believe older boats were generally tougher built, perhaps through ignorance, as they could not calculate the critical loads, so didn't bother. Rather reminiscent of Victorian engineering - just look at the London pumping stations still going after a century whilst the modern ones have to be regularly replaced.
I would happily have either type of keel, but there are different considerations in how to look after them.