Timing is actually also based on patterns and anticipation, rather than it just being based on a physical ability (reaction time). Most of the time, when a boxer manages to time his opponents with perfectly placed punches, they are usually doing so after figuring out their opponent's movement, which tends to be performed in a specific pattern. It's pattern recognition skills! For example, when Canelo Alvarez and Danny Garcia knocked out Amir Khan, they did so by anticipating what punch he will throw in advanced and counter punched accordingly where and when the opening was. In fact, Garcia's eyes were practically not even looking at Amir Khan when he caught Khan with that punch.