It's a good thing I have stuff like this saved for #help.
PP is based on relative skill. The harder the map, and the better your play on it, the more pp you gain.
The pp that you gain from a play on a map is the 'raw' pp. This 'raw' pp is the same as Tillerino tells you that you would get.
Now, for the sake of explaining this, let's say the raw pp is 50pp.
That play gets in your top ranks, and is then weighted against all your other top ranks. And you would get for example 35 'weighted' pp.
So you didn't gain 50 pp, and were never meant to.
Furthermore, because that play is in your top ranks, and is weighted, everything below is now weighted less than before.
Meaning that you gained 35pp, and then lost Y pp because of everything below it now being worth less.
Which makes you effectively gain a small amount of pp (for example 1 - 10) instead of the 'full pot'.
Either how. You can't just keep piling up pp. It's based on relative skill as I said at the beginning.
At one point you will cap out and cannot gain pp anymore if you keep playing the same difficulty (at the same skill level).
Ranking is based on pp. And people constantly gain more pp.
So you to keep climbing in ranks you have to gain more pp on avarage compared to the 'natural flow' of people climbing ranks.
TL;DR
Play harder maps and do better on them in order to gain pp.