I wouldn't say that there is a clear manipulation, we don't know if the data is that accurate in the first place. Assuming it is, gamblers though from time to time shift casinos too so maybe in the next released of the data it could be very different result.
As for Stake, they are one of the first, the prime mover in the crypto base gambling platform. So there will be no surprise that they occupy the top spot. Similar to Bitcoin or Ethereum as prime movers in crypto as well.
Since
Tanzanite the analytical company who came up with the report is supported by Stake and Yolo members might assume that this data might have some error. But Tanzanite claims that they are independent and their financiers don't influence them.
With the amount some of these casinos spend on promotion, the list might have some level of validity. Some of them pay millions of dollars for endorsement deals from top sports clubs and well-known influencers.