Down Bag weigh gain

I have a TNF down 20 degree bag("Orion") about 15 years old. Have washed it twice, most recently 3-4 months ago. Curious, I weighed it and found it was about 18 ounces heavier than the original, listed weight; this was the first time I had ever weighed it. I have always worn clothes while using it.Thinking it might be moisture, I put it in a dryer it a week ago for about 45 minutes; the weight was unchanged. Can't quite figure this out.