Rex puts his all into dodging the attack, his movement is motivated by a plan inside his head.
It was always his trait to take an attack head-on.
Many would think that this unique trait of his would be the end of him one day and is outright stupid, as the safer way of approaching this would be to dodge an attack that has a chance to hurt him, no matter how small that chance would be.
But Rex knew that he would only break through his limit if he was under immense pressure.
His potential only shines when there's a huge pressure threatening him.
Additionally, this trait would allow him to gauge the very limit of his current capabilities.
Unlike normal people or even Awakened that was vulnerable to long-term damages they sustained in a battle, Rex has no such problem as his regenerative capability would always heal him back to normal as long as he has sufficient strength.