You will need to work harder to earn PAI points as your PAI score increases. It is much easier to earn PAI points when your PAI score is low, say below 50, than when your PAI score is higher, say above 100.
The exact same exercise will earn you a different number of PAI points depending what your PAI score was when you started.
It's also important to remember that reaching 50 PAI is still a great achievement; you'll receive 60% of the heart health benefits with 50 PAI, and 100% of the benefits by earning 100 PAI.