The signed error convention matters around the 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
Input below the returned level gives negative error
Input 9/2 V minus reconstruction 5 V gives error -1/2 V.
29V−5V=2−1V
Input at the returned level gives zero error
Input 5 V minus reconstruction 5 V gives error 0 V.
5V−5V=0V
Input above the returned level gives positive error
Input 11/2 V minus reconstruction 5 V gives error 1/2 V.