No, a +1AC shift = 5% change is absolutely true.
Situations like this only occur because gaming systems allow you to have a greater than 100% probability of success (ignoring the 5% chance of failure on a roll of 1 on a D20)- a statistical impossibility.
Autofailure aside, the 5% shift is lost in the noise of modifiers that allow the probability of an event's occurence (here, a successful attack) to be greater than 1 (absolute certainty).
In other words, while the 1 point shift IS absolutely a 5% shift in probabilty, the fact that the system allows for probabilities in excess of 100% overwhelms the normal math.
Situations like this only occur because gaming systems allow you to have a greater than 100% probability of success (ignoring the 5% chance of failure on a roll of 1 on a D20)- a statistical impossibility.
Autofailure aside, the 5% shift is lost in the noise of modifiers that allow the probability of an event's occurence (here, a successful attack) to be greater than 1 (absolute certainty).
In other words, while the 1 point shift IS absolutely a 5% shift in probabilty, the fact that the system allows for probabilities in excess of 100% overwhelms the normal math.