Is my GROUP BY lying to me?

Any aggregation that feeds a number somebody will act on.

Review the aggregation only. First tell me what one output row represents. Then check: does a non-aggregated column vary within its group, are zero-row groups missing from the output entirely, and is anything averaging a ratio that should be a ratio of sums? Show the corrected fragment wherever it is wrong.

Query:

Correctness checks. Catching the join that duplicates rows, the NULL that drops them, and the GROUP BY that lies.