Azure Advisor aggregates scores across multiple subscriptions by calculating a weighted average based on the number of resources or services contributing to each pillar within each subscription, rather than a simple arithmetic mean. This means the score for each pillar is influenced by the proportional contribution of resources from each subscription to the overall metric. For example, if Subscription 1 has significantly more resources impacting the "Cost" score compared to Subscription 2, its score will carry more weight in the aggregate calculation. This ensures the aggregate scores reflect the real-world impact of recommendations across all resources rather than treating subscriptions as equal regardless of their size or complexity.
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hth
Marcin