Changing one-row strip length makes the projection sum source visible. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Two cells give row sum three
A one-row strip with cells 1 and 2 has line sum 3. This strip has no diagonal measurement.
1+2=3
Three cells give row sum six
Adding a third cell 3 raises the same one-row projection to 6.
1+2+3=6
Four cells give row sum ten
The four-cell strip adds 4 and the row projection becomes 10. This is still exact strip arithmetic, not a full scanner model.