osu! Wiki wrote:

The weightage system refers to the fact that all of your scores are weighted against each other in terms of overall performance.

This means that only your highest pp score will award the full amount of pp for that particular performance. Other scores with lower overall pp values will be weighted with a gradually decreasing percentage.





To provide an example of this with a mathematical formula:

Let PP represent each score's individual pp value. PP[i] denotes the _i_th score's pp value, sorted descending, where i ranges from 1 to n, and n is defined as the amount of scores you have.

Total pp = PP[1] * 0.95^0 + PP[2] * 0.95^1 + PP[3] * 0.95^2 + ... + PP[n] * 0.95^(n-1)

osu! Wiki wrote:

Why didn't I gain the full amount of pp from a map I played?

Performance points use a weighted system, which means that your highest score ever will give 100% of its total pp, and every score you make after that will give gradually less.

This is explored in depth in the weightage system section of the article above. To explain this with a simpler example:

If your top pp rankings has only two maps played, all of which are 100pp each scores, your total pp would then be 195pp.

The first score is worth 100% of its total pp as it is your top score.

The second score is worth only 95% of its total pp as it is not your top score, so it contributes only 95pp towards your total instead of 100.

Now, let us posit that you set a brand new 110pp score. Your top rankings now look like this:

110pp, weighted 100% = 110
100pp, weighted 95% = 95
100pp, weighted 90% = 90
As you may have figured out, your new total pp is not simply 195 + 110 = 305pp, but instead 110 + 95 + 90 = 295pp.

This means that as you gradually improve at osu!, your pp totals will trend upwards, making your older scores worth progressively less compared to the newer, more difficult scores that you are updating them with.