When starting at zero PAI—or at a PAI score below 50—it will be easier to earn PAI points compared to when you are at a higher PAI score, say above 100. As you gain more and more PAI over the course of the 7 days, the algorithm adjusts to your improved fitness level. However, exercise still needs to be maintained, otherwise its benefits expire. If you don’t workout for 2 weeks and your PAI score drops to zero, it will be like starting over again.
Your heart is a muscle, and like all muscles, it gets stronger each time it repairs from its previous workout. For a quick way to remember why PAI becomes more difficult to earn, remember that your heart is becoming stronger (albeit slightly among each instance) and the PAI algorithm must also adjust accordingly.
It's also important to remember that reaching 50 PAI is still a great achievement; you'll receive 60% of the benefits towards living a healthier life with 50 PAI, and 100% of the benefits by earning 100 PAI.