AdWords Service Adjustment

Just noticed a "AdWords Service Adjustment" in my AdWords account in the billing section. I've seen plenty of "Click Quality" adjustments but this is my first service adjustment. It's for over $400 too!

When you mouse over the question mark next to it here's what it tells you a service adjustment is:

"A credit applied to your account indicating promotional credits, courtesy credits, and other similar billing adjustments. "

I'm not aware of any promotional credits or billing adjustments to this account...maybe I received some courtesy credits? I'm sure this isn't new, it's just new to me.

I found this in the AdWords help center: service adjustment.