It looks like the aggregate stats are more of a venn diagram than an average. So if 1/N services are down, the aggregate is considered down. I don't think this is an accurate way to calculate this. It should be weighted or in some way show partial outages. This belief is derived from the Google SRE book, in particular chapters 3 (embracing risk) and 4 (service level objectives)
https://sre.google/sre-book/embracing-risk/
https://sre.google/sre-book/service-level-objectives/