Given how much simulation they do ahead of time, especially if the chip is lower in CPU power (less expensive to simulate) I would actually expect them to be able to hit their targets/promises.
It's a reasonable expectation, but I've seen a few different chips tapeout and all of them missed their performance targets by varying amounts. There are just too many things that can go wrong, and unfortunately quite a few of them usually do.
You're right that the low-power ones probably have more accurate simulations than the high-end ones.
You're right that the low-power ones probably have more accurate simulations than the high-end ones.