It does this because the browser tries to pre-empt the image loading, even though we then discard the Tweets after the 3rd. We’ve got plans to fix this, both in terms of managing data-tweet-limit fully on the server (so it won’t return extra Tweets to be discarded at all) and also in terms of not exposing image sources to the browser before they’re finalised.
Some of these fixes should be out in the next few weeks.
Ben