The counter-intuitive behaviour trips up most reports: when a page starts ranking for hundreds of new long-tail queries at position 30, the average position gets worse while traffic grows. Conversely, losing visibility on a mass of weak queries improves the average. Read it alongside impressions and clicks, never alone.
For monitoring, segment before averaging: by query type, by template, by country and by device. A single site-wide number is close to meaningless, and it is exactly the number most agency reports lead with.