A pileup bucket sums hits that land in the same local cell window from different sources, so the local count can exceed the single-track expectation. The bucket total of 3 plus 1 equals 4 hits exactly for the stated local sum; real pileup adds dead or noisy channels, threshold drift, pileup ambiguity, clustering choices, alignment error, readout inefficiency, and finite-count uncertainty.
A local bucket is an exact finite hit sum. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A bucket can add nearby local hits
One bucket receives 3 hits from one side and 1 from the other.