Range of estimates on how many seats Republicans will pick up in the House

Right now Democrats have a 37 seat majority, though the two vacancies were previously held by Democrats. On average these estimates imply a Republican majority of 12 seats, or a net pick up of 51 seats. All these projections have Republicans picking up seats, but the least optimistic one has them only picking up about six. Real Clear Politics has them apparently picking up 74 (35+39) (at least that is what this figure from the WSJ claims). The WSJ claims that it got these estimates from PollyData, but I am not sure that I can match all the claims shown in the figure.