We all - the OSE donors - are donating personal savings to make this work, and are directly interested to make this org as efficient as possible. Having skin in the game is best way to keep such nonprofits accountable. There is no leftovers or fees - all investment income from donations goes to open source, except for minimized operating expenses (e.g. accounting). It is run by the team of volunteers without salaries, and we require $1000+/year donations from all directors of this org.

Are there rules on where you park the money between when you get it and when FOSS gets paid?

The README has a 2-3% gap between expected returns and outlays, surely that is not all going to accounting?