It might have something to do with your LeftOuter Join kind ("join kind=leftouter")
From what I know it means that if nothing is found on the right table join then a nothing is put in place.
leftouter In addition to the inner matches, the results include a record for every record on the left table, even if it has no match. In that case, the unmatched output cells contain nulls. In the given example, 3 records on the left ("exceptions") table don't have a match on the right ("requests") table, and therefore have an empty "avg_dur" column.