Projection data can be insufficient even when every shown sum is exact. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Row and column shadows can match
Two different grids can share row sums 3, 7 and column sums 4, 6.
3746
A matching shadow is not yet a unique image
The accepted bit is 0 until an independent measurement breaks the tie.