Your software can only guarantee what the hardware provides/guarantees. If your cpu can take a variable amount of cycles to start processing an interrupt, the RTOS can only guarantee the upper bound. A real time cpu is all about being faster and more importantly more deterministic, therefore enabling the RTOS to guarantee faster reaction times.

If an interrupt can usually be processed in 50 cycles but very rarely will take 500, the RTOS can’t schedule for the 50 cycles. If the real time cpu guarantees 200 cycles, the RTOS can actually schedule with that. Depending on the application, that can make a huge difference.