You're assuming their business plan included only the one round of funding. I'm assuming they were planning extra rounds or the VCs believed they would need extra rounds.
If they went up to, say, a Series B at 10M and a Series C at 20M, which are reasonably conservative multiples, they are at ~36.5M, so looking for a ~400M exit. It's not hard to need that ~1B exit if your business plan is "everyone in the world using my product," and it appears this is what Everpix were aiming for.