Who are matching funds in quadratic voting and what are their incentives?

Let's take an eg,

Say the total donation pool in quadratic funding is 10k, for 2 projects, project A receives 6k by 50 ppl while project B receives 4k by 150 ppl

Ultimately project A would receive 24k and project B would receive 17k roughly speaking

Now the original donation pool is 10k, then who's gonna pay the remaining 31k?

I've informed that it's the central matching funds; so like they are paying out of their pockets for the projects? Who are they and what are their incentives for doing this?