The admin fees you refer to are typically assessed in the context of large, institutional grants. These are grants from private organisations like the Gordon and Betty Moore Foundation, the Alfred P. Sloan Foundation, the Chan Zuckerberg Initiative, or government agencies like the US National Science Foundation. These grants have significant financial accounting requirements. There are also many other legal or operational costs associated with these grants.

When projects run these grants through parent institutions like universities, the typical admin fee is >40%. In some cases, it can be as high as 60%. Many projects are eager to enter fiscal sponsorship agreements with organisations like NumFOCUS, because so much more of their grant money goes to funding the work.

In the case of NumFOCUS, admin fees do not adequately cover the staff requirements to manage these grants. NumFOCUS "loses money" when servicing the administrative needs of these grants & it takes this responsibility onto itself solely for the betterment of the projects.

Rather than assess administrative fees similar to universities, NumFOCUS uses its other fundraising—corporate donations, event (i.e., PyData) sponsorship, individual giving—to finance its operations.

source:

- I serve on the NumFOCUS board of directors as its co-chair.

- I presented on this topic last year at the NumFOCUS Annual Summit to an audience of core developers from projects like Julia, Jupyter, Pandas, NumPy, AstroPy, &c.

- NumFOCUS budgets are public, and all of the above information can be corroborated from materials published on https://numfocus.org/