It's the standard DO credit email, which typically is from referring friends or doing something else to acquire credit. Not getting a refund because DO accidentally deleted your droplet.
Though their should be another avenue for such things.
Hmm, maybe that's what they're thinking. But I wonder if it's really true that a refer-a-friend or similar, happy scenario is the "typical" cause for a refund. Certainly in my life I've received far more refunds due to company mistakes than than credits due to the happier reasons you mention.
And when in doubt, if you're refund system is not capable of discriminating between the two, it seems to me wiser to err on the side of a less jubilant message.
DO credits from positive events are pretty common in my experience; not just refer-a-friend, but they give out credits through ads on podcasts and the like, through promotions for students and whatnot. I think I've had something like $150 total gifted DO credit.
Though their should be another avenue for such things.