The obstruction rule is a rule of equity. It exists to right a wrong and place the players into a position that they should have been in, in the umpires' judgment, should the obstruction not occurred.
Umpires often have to make immediately decisions in their minds, though may have time to actually make the call.
If an umpire makes an immediate determination upon obstruction that he is going to protect the runner to 2nd base, but later sees that the runner gets thrown out by a hair at 3rd base, he must award the runner 3rd base. An umpire's inaccurate immediate determination of the protection shall not place a runner in jeopardy. Neither the defender's illegal position on the field nor the umpire's original poor judgment can prevent a runner from obtaining the base she should have obtained absent the obstruction.
In reality, how can anyone know that an umpire changed the extent of the protection unless an idiot umpire verbalizes such? "You're right coach. Your runner was thrown out by an inch at 3rd base. However, when she rounded 1st base, I made an immediately determination, and quite obviously an inaccurate one, that I would only protect her to 3rd base. I failed to take into consideration all of the factors when making that immediate determination. I hope you accept this explanation and don't mind me calling your go-ahead runner out at 3rd base to end the inning. Please, next time, assume I will make a poor decision in my head that only I can know about and hold your runner at 2nd base."
I think that some people are failing to recognize that "in the umpires' judgment" requires the umpire to take into consideration many factors. Failure to do so immediately doesn't change the intent of the rule nor prevent a just ruling on the field."