The local results are:

Mapper	Combined records
1	(cat,2),(dog,4)
2	(fox,10)
3	(cat,6),(fox,3)
4	(dog,5),(fox,7)

Correct reducer groups: cat:[2,6], dog:[4,5], fox:[10,3,7]. The keyed c gives fox:[10,10], apparently merging 3+7 across mapper boundaries. That is not the local-only operation stated. It happens to preserve final counts 8,9,20, so it is not a wrong final reduction, but it is a wrong asserted input trace. The example is retained because it teaches both combiner scope and how to handle an inconsistent official key.

A combiner can run zero, one or several times, so an aggregation must remain correct under regrouping. Addition and maximum are associative and commutative over ordinary exact arithmetic. Average-of-averages fails with unequal group sizes; (sum,count) combines componentwise and divides only at the end. Floating-point addition has rounding-related nonassociativity, but that is not the issue tested by this bank.