I think your explanation is very likely the correct one, Peter. By using domed spring steel washers (Belleville washers), and tightening the nut until they have squashed a bit, means that the odd torsional impact caused by hitting a concrete edge or two with the cutter, does not make them lose driving torque afterwards. With a mower, hitting large pieces of concrete with a bar blade is hopefully rather uncommon, but it happens constantly with an edge cutter, at least when I'm driving it.
