I mean Microsoft only has 2561 members (https://github.com/orgs/Microsoft/people) , so it isn't how many microsoft members contribute to open source.
Is it how many contributors their open source projects have? nope, FontAwesome has maybe 100 contributors to all of it's 5 projects but it's listed as having 9000+ contributors.
From what I can gather it's mostly based on the number of people that forked one of their projects (with some padding, maybe by the number of contributors to forks of the project? I have no idea)
This doesn't seem like a metric that is more meaningful then just the number of stars a repository has.