What I tried to say is, a mainframe might answer late, but rarely, and very slightly. a RTOS doesn't deviate from that number in either direction. It's neither early nor late.
Some RTOS even doesn't consider late answers as acceptable.
That's true, but that jitter envelope gets bigger as the load increases. For a mainframe this jitter envelope is much narrower, and in a competent RTOS, that's basically 0.
Some RTOS even doesn't consider late answers as acceptable.