I think most of you are reading it "backwards."
What the marking on the breaker terminations means is that the wire needs to be sized using the 75 degree column ... and not the 90 degree column ... in the tables.
If you had a 60 degree wire, the result of using the 60 degree column would be a larger wire, not a smaller one.
More important, the 75 degree is an improvement; in former times, we were limited to a 60 degree limit to the terminations.
Think of it as a "chain," we can only load up the wires based upon the 'weakest link,' or lowest rating, in the circuit.
Or, looking at it from another direction, that 75 degree termination means that THHN -a 90 degree wire- is limited to 75 degrees in your calculations.