Ian,
Tire size can be a factor also, so one needs to know whether the measurement was done with Thorn's default tire for the bike in case you might decide to equip yours differently -- can be important if clearance is tight.
When I was doing my research prior to buying my Sherpa Mk2, Rachel at SJS Cycles said the default tire for its measurements was 1.5in vs 2.0in for others like the Nomad.
Might be worth dropping a note to SJS to confirm, as Dave Whittle might not see your query here.
All the best,
Dan.