To prevent skilled players from grinding hundreds of easier maps worth less PP to rank up, a curve is applied to your plays. This curve is 0.965^(x-1) where x is the position of that score on your ranked map list which can be found on your profile.
You might have noticed that in your list, there are 2 numbers which end with PP. One of these numbers is blue and represents the raw amount of PP you get from that play, before the curve is applied. The second number is gray and in parenthesis showing the actual weighted amount of PP you are getting from that play after the curve is applied.
As said before, the amount of weighted PP you will receive is determined by the curve and the placement of that song in your list. As an example, if your top play is a 260 raw PP play, you will be rewarded 260 weighted PP which is a 100% of the raw PP of that play. If your second top play is 247 raw PP, you will be rewarded 238 weighted PP which is the actual amount of PP you get after the curve is applied on it.
Here is an example of how the curve works:
Raw PP
Weighted PP
260pp
260pp (100% of raw PP)
247pp
238pp (96.5% of raw PP)
246pp
229pp (93.1% of raw PP)
244pp
219pp (89.9% of raw PP)
A common question people seem to have when they get a new score worth, for example, 100 weighted PP is why they are not getting +100 pp on their total amount of performance points. As explained earlier, since the effect of the curve on your plays increases the further you go down your list, when you make a new play, it takes the place of an older play which makes all the plays that are under go down 1 place in the list. Even if you are getting +100 pp from this new play, all your lower plays are weighed down even more than before so you lose a certain amount of PP on them. This is why when getting a new play or making a better play on a map, you will never (one exception) get the same amount of PP on your total amount of performance points.
The only exception to this is if you get a better accuracy on your current top play. Since your top play is not affected by the curve, meaning that none of your songs will change places in your list, there will be no lost of PP on your other plays. An example would be if your top play is on Avalanche with 250 pp and you manage to get a better accuracy and that play is now worth 260 pp, you will then get +10 pp on your total amount of PP.