Why recipes measure ginger in 'thumbs' instead of grams

Food writers say recipes list fresh ginger in thumbs or centimetres mostly by convention, since dishes like curries rarely need precision and exact weights can make cooks anxious. Felicity Cloake admits grams would be more accurate given ginger's varied shapes, while Ravinder Bhogal defines a thumb as roughly a 5cm piece, the length of an average adult thumb.