Fitness Points are earned based on your current Fitness Age and VO2max, and the intensity and duration of your activities. Although you are doing the same type of activity, your heart rate might behave different over activities on different days. For example, you might not be feeling well, you might have just started training after not training for a while, or there might have been a strong headwind. The times spent in higher heart rate zones earns you Fitness Points faster.
As you get fitter, the same type of activity, for example your 5k runs at 10 km/h, will earn you fewer Fitness Points than earlier activities. You'll need to increase your distance or pace to earn the same points.
There are also diminishing health benefits within activities. This means that it’s better to do short and intense activities a few times each week, than one long activity. For example, the first 10 km of a longer run might generate points faster than the final kilometres.
You also can't compare your Fitness Points with another person's points because everybody will earn points at a different rate. For example, when you are fit and have a low Fitness Age, you'll have to work harder to get the same Fitness Points as someone who is just getting started. So as you get fitter, then the harder you'll have to exercise to earn Fitness Points.