I'm curious, do they truly give it all away, or is there something more going on behind the scenes?
Yes and there is every possibility for such happened considering how wealthy they are or could be, people like Elon Musk, and Bill gate could do that because they makes millions dollars within 24hrs and if they keeps spending that wouldn't affect their overall savings or investments reason because they already has investment where they are already reaping it folds, so they aren't much cared if they spent that on their foundation. What they believe is the life they would have to impacts before finally leaving the face of the earth.
I would consider they would only give it away at the end of their life if they want to. And there is a HUGE difference between Bill Gates and Elon Musk. Bill Gates has been rich since he was nearly a teenager basically, very young, and grew up being one for the richest people in the world and that means he had time to sink that in.
Elon Musk has been working to get here, remember, he had just some millions after paypal thing, and was putting all of that into other companies, only after Tesla took off that he became really rich, and we have seen what that made him do, so he is a bit of a weirdo, I mean he is on scale for sure. That is why, Bill may give it away, he already probably guaranteed all his children's future, but Elon may not, he is not the kind of guy who would like to help anyone.