I'll try my best to explain it to make it seem as simple as possible.
Let's say you don't have a score on a map. It's going to give you 0pp and be weighted 0%. Should be obvious.
Now you play that map and get a score that's worth 67pp and is weighted 54%. This play is now weighted to be worth 36pp. Weightings make it so your better plays are worth more than your worse plays, and keeps it so that each player has the same number of scores that give pp so you can't just play every map and continue gaining 67pp from every play. That should also be obvious.
But why did you only gain 5pp? That's the part you're missing. When you get a new top play that's weighted 54%, the play that used to be weighted 54% isn't as good, so now it's only weighted 51%. Let's say you had a play that was giving you 66pp there before. It was giving you 35pp after weighting 54%. Now it's only giving you 33pp. What about your 64pp play that used to be weighted 51%? Now it's only weighted 49% and gives you 31pp instead of 32pp. And repeat that for all of the scores on your profile.
All of the scores that you beat are now considered to be not as good, so you lost 31pp from all of those scores being weighted less. But you gained 36pp from adding in the new top score. That results in a net gain of +5pp on your profile. You'll always gain pp when you improve a score or get a new top score like this, but you'll never gain the full amount that the play is worth.
Hope this helps!