The BMGF operates through endowment, which a lot of organizations do. Basically, they don't expect many regular small donations (like a lot of smaller charities do), they expect just a couple of very big ones. Because the donations don't come every year, they need to last. So they invest the money, and use the interest for operation costs and grants. This way, they can give that $2b a year[1], in perpetuity. Another big donation means that they only give out slightly more per year, but that the donation will basically last forever.

There may be a better model, but this is a well respected method for making a big charitable gift last.

[1] more in years when the economy does well. E.g., in 2009 they gave slightly over $3B.

Giving $2b/yr in perpetuity is definitely not the mission of the Gates Foundation. They have set it up to run out of money within X years (10 IIRC) of Bill and Melinda's death.

Did you just equate 5% (close to a standard annuity rate) with 0.4% (close statistical noise)?