I get what you're saying and it makes sense, however, this is not a new thing that is only a few weeks old. Bran has been at the top for months now in multiple parlors. While some of that could most definitely be attributed to what you mention, I don't think he would have this type of 6-10 month consistency at the top if it was based on speculative betting only. Yet, no one else has ever caught him in that time period in these odds.
It looks like more than speculative betting to me, simply because considering the nature of the character and how unimaginable him coming out on top seemed at the conclusion of season 7. It's hard to imagine that the amount of people and/or amount being put down on him, is being done totally in the dark with no potential insider knowledge. There's been 8 years to bet on Bran in these kind of entertainment bets, and he's suddenly exploded to the top of heap in only the last 6 months which just so happen to fall right before the final season. Seems like an awfully big coincidence.