Newman,
Everybody is talking about adds, but nobody mentions cancellations. These are Net adds.
Now this is entirely speculation on my part. But there must be a large number of people that have (had) subscriptions to both services. With the advent of the "Best Of", it may be than a number of these people have, or are expected to, cancelled one service and have just added "Best Of". Is that a run on sentence?
It's a possible answer to your question, even though I have absolutely no data to back up my speculation.