The bookies are probably factoring in the Ex Pat votes which appear to be all for staying and the Scottish vote which is also for staying. I doubt those two groups appear in the published polls and you are looking at a possible 4 million stay votes not accounted for. As you say, the bookies are not out to lose money.
Generally speaking the Ex Pats only care about whether they can stay in their chosen bolt hole and continue to draw their pension. They couldn't really give a toss about the strain on our infrastucture that unlimited immigration will bring because they feel they balance the figures. We could be going to hell in a handcart for all they care. If they thought much about Blighty they'd still be here wouldn't they. It's all self focussed.
The Scots are in a quandary. Do they vote tactically to bring about the conditions for a second referendum or remain true to themselves? The bookies think they will remain true to themselves but I wouldn't bet my house on it. Canny people Scots.