GM: Free Int'l EC?

I was in a 'preferred' seat on the international legs of my upcoming trip until two days ago. Today I see that on one of the legs I am assigned to an EC seat. There's also a $0 receipt generated for it. I called the GM line up and asked to confirm my seat numbers, and they confirmed the EC seat number.

I wonder how this happened. Glad that it did, and also wishing it happens on the other legs, but oh well :p

Does this happen every now and then?