That's because those are the four teams expected to be in the CFP, so they are the top teams likely to win the title (have to be in the CFP to win the title.). Vegas knows full well that Bama is given a pass to the CFP barring a complete collapse, unlike other teams who aren't given that consideration.
Perfect example is 'Bama being in the top 4, above Penn State the same week that Clemson was put below Penn State...based on the supposed argument that Clemson hadn't done anything this year, hadn't looked impressive, and Penn State had better wins. Meanwhile, 'Bama hadn't done anything yet this year, hadn't looked impressive, and Penn State had better wins, but of course Penn State didn't jump 'Bama.