I think you’re calculating wrong
For Trump to go from 5:1 to 2.5:1 is major swing
5/1 is 16 %. 5/2 is 28 % (roughly). I haven’t checked in a while but last time I did I saw they were offering 3/1 (25 %) on him. They’re no doubt adjusting a bit because some money is coming in on Trump. I guess what to make of it is all subjective.