Deterministic matching is auditable. Every merge in the graph can be traced back to the event that triggered it and the identifier that linked the two profiles. Probabilistic matching produces links based on statistical similarity — same IP, similar device fingerprint, overlapping browsing windows — and when an audit or a regulator asks why two profiles ended up merged, "the model thought they were the same with 87% confidence" is not a defensible answer. Industries handling regulated data generally need the auditable version. The trade-off is reach: probabilistic methods catch links deterministic methods miss, particularly across devices when nobody has logged in.