Right, because that’s something you’re expecting. If, for example, a rein snuck up behind you and you didn’t realize they were there and tried to shatter you, it’s pretty reasonable to assume you wouldn’t even come close to blocking it.
The reason a lot of high ranking players tend to have or appear to have such good reactions to things is not because of their inherent raw reaction times, it’s that they’re capable of predicting behaviors and interactions and can prepare for them.