A conductor usually has two termini. In the case of the tap conductor under discussion, one is at the tap and the second is at the breaker.
Let?s assume the tap termination is suitable for 90C and the breaker termination is 75C. Let?s further assume ambient and other ?conditions of use? do not enter into the analysis.
At the tap, the "conditions of use" permit the tap conductor its 90C rating. At the breaker, the "conditions of use" limit the tap conductor to the 75C rating. Since the current can?t exceed 65A, what is its ampacity as defined by Article 100? Well IMO it?s 75A at the tap and 65A at the breaker. (BETWEEN the tap and the breaker termination the ampacity is 75A under that ?condition of use?)
Since the breaker limits the tap?s current (NOT ampacity) to 60A, it is adequately protected.