To be able to GAIN points in a certain stat, you have to LOSE points in another.
So you decide to start levelling Jumping. Agility is the primary stat for Jumping, so after 2 hours of jumping, AGI is increased by 0.1, and some other stat (example: Strength) is decreased by 0.1.
But because the stat window only displays whole numbers, you see Strength decrease, but no corresponding INCREASE in Agility.
After 10 hours of non-stop jumping, Agility is at x+0.5, and Strength at y-0.5 (stats window shows no change).
After 20 hours of non-stop jumping, Agility suddenly shows an increase of 1 point in the stats window.
That's my current theory anyway