My wife got comped on NW for having Marriott elite status. Also gotten it the other way - Hyatt and Hilton due to CO status. She had even called once about getting bumped to Plat early on CO, so she could get lounge access on an Int'l coach flight. CO seemed willing, but when she told them that the flights were NW, they balked - they want to see confirmed bookings on CO that will earn the mileage.

I'm guessing that CO saw your pattern and future tickets and thought it would be a nice gesture. Good for them, but not unprecedented.

My suggestion would be to call prior to the gold-earning flight to see if they will accelerate you again. The worst they will do is say no, and if they say yes, you can earn some extra bonus miles.