Physios know as we are trained to take into consideration length of time for various tissues to heal eg bone, tendon etc. Even then it can still be a rough estimate depending on the type of injury/surgery and how the person responds.
In addition if the person concerned has any preexisting medical conditions such as diabetes this will affect healing and recovery time. That can also be said for preexisting problems with say a joint that has been injured before. return to playing dates should not be set in stone and a player should never be cleared for playing until passing all relevant fitness tests without significant reproduction of symptoms or ill effects.
Take a read through Dandy and Edwards core texts on healing