Why are stuck-at faults more efficient when applied at the circuit’s rated clock?

Timing failures can occur when a circuit operates correctly at a slow clock rate, and then fails when run at the normal system speed. In other words, when stuck-at fault tests are applied at a slower clock rate some faults which are a manifestation of timing failures could not have been detected. Such faults, if present will fail the chip ultimately. Hence At-Speed testing which is nothing but applying tests at the rated clock speed could detect such chips which are bound to fail due to timing failures.