I think that most of the 45K instances are small experiments that only get a few HTTP requests per day. I would guess that in this case an instance that does not get a request for a while that it gets 'swapped out.'

I have no firm data, but a customer last year put a very low volume web app on Heroku, and I guess this is exactly what we saw: the first request after a quiesant period took many seconds - then subsequent requests were quick. If I am right about this, customers with high volume web apps would not notice this.