Lower-edge reconstruction makes the error visible as input minus returned level. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
One-volt steps leave half a volt
Input 11/2 V with code 5 and LSB 1 V reconstructs 5 V, leaving error 1/2 V.
5⋅1V=5V,211−5=21V
Half-volt steps land exactly
Input 11/2 V with code 11 and LSB 1/2 V reconstructs 11/2 V, leaving error 0 V.
11⋅21V=211V,211−211=0V
Another between-edge input leaves the same half volt
Input 7/2 V with code 3 and LSB 1 V reconstructs 3 V, leaving error 1/2 V.