I think the extra DEC you got is from one of your cards selling on the market and the person paid with DEC. It is now possible to buy cards with DEC which to me is kind of annoying. This happened to me the other day. I was playing and all of a sudden my DEC jumped up by 800. At first I thought that I got 800 free DEC from some glitch. But later I figured out that one of my cards sold and the person paid with DEC.