The best way to think about getting the rash and urushiol oil is as a "Get Out of Jail Free" card. You get to use the card X amount of times and then its not there anymore. The first so many times you come in contact with the urushiol oil, you didn't have a reaction but you did become sensitized to it. Then one day, it's there - the rash we all dread. It's not that the pi is any "stronger" its that you are now allergic (like about 85% of the rest of the population where these plants reside).
Here are some references: