One guess that I'd have is that a16z is looking for synergy. That means they're not really thinking of Atrium as an isolated home run investment. Instead, they're looking at them as solving a significant issue for the companies they invest in. If a16z sends a stream of business their way, they'd not only get the $65m, but a lot more in business. And from a16z's perspective, Atrium could solve a major headache/risk factor for early stage companies, so $65m is a small price to pay for all the returns they get from their other investments, even if Atrium never exits.

And if there is an exit, it's the best of both worlds.