Your first query was getting the count of distinct Incident_id, while your second query was getting distinct combinations of Incident_id and policy_name. Surely you can see those two are not the same. They will only return the same count if every Incident_id only has one policy_name associated with it.
Now, which of the 2 cases are you looking for? I thought it was obvious you were looking for the second choice, but I'm not going to assume.
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