An organization page answers a narrower question than it might look like: how much have this organization's public members publicly contributed to the organization's own repositories? Every number on the page — the headline stats, the member leaderboard, the organization's position on the organization leaderboard — is built from that one measure.

The calculation

For each public member of the organization we ask GitHub for their contributions scoped to that organization — commits, pull requests opened, pull-request reviews, and issues opened, over the member's whole lifetime on GitHub. Only work in repositories owned by the organization counts; a member's side projects, work in other orgs, and personal repos don't.

Within that scope, GitHub's usual contribution rules apply, the same ones your personal profile uses: commits count on the default branch, under an email linked to the account, in non-fork repositories.

The organization totals are simply those per-member numbers summed. The member leaderboard shows the same rows the sum is made of, so the two always reconcile.

What the number is not

It is deliberately not "everything that ever happened in this org". Three things are excluded, and they can make the totals smaller than you'd expect:

Private members. GitHub keeps organization membership private by default, and we can only see members who made theirs public. An active organization whose people all keep membership private shows a total of zero — see why am I not on this member list?.
Outside contributors. Someone who sends pull requests to the org's repos without being a member isn't counted. The measure is about the organization's people, not its repos' traffic.
Private repositories. We only see public activity. Work in the org's private repos is invisible to us, even for public members.

So treat the organization numbers as the org's public footprint: what its publicly-listed people have visibly built there.

How this differs from a personal profile

Your personal page counts your public contributions across all of GitHub. An organization page counts only what landed in that org's repositories. That's why your number on a member leaderboard is always less than or equal to the number on your own profile — often much less, if most of your work happens elsewhere.

Freshness and quirks

Totals cover every completed month up to now; the current in-progress month rolls in once it ends (the same convention as personal profiles).
An organization's numbers are collected when it's first looked up and refreshed periodically after that, so very recent activity can take a while to appear.
Organization pages don't have a commit chart yet — we currently store lifetime totals per member, and the per-month history that powers the chart is collected in a slower background pass.
In rare cases GitHub's API cannot serve a specific slice of a member's history and we have to count that slice as zero. This is a GitHub-side limitation and typically affects a single member's older years.