Well, I do have the time, but it doesn't require any in-depth explanation as to why that is normal. It's just pure math, or rather formulas. One VERY important aspect of it is accuracy, and if you don't nail that song like a common brothel prostit- nevermind, just remember that you need to do really really good (almost flawless) in a hard song to actually milk all of that pp out of it.
One good example is your "Rabpit - Saika" map play. You got more pp out of the Hard difficulty (23 if my memory serves me well) , than you did on the on the Expert one (19)... why? Well, you did really, REALLY bad on the Expert one in terms of accuracy... you also got a B score for it due to all the mistakes... so it's simple - if you want to advance, play songs which are just a tad BIT over your top score's difficulty in terms of star rating, and FC them with not less than 97% acc, and you'll advance really really fast. Don't bother playing insanely hard maps with mods that make it easier, but rape your score, and still fail on them.
The reason I haven't given you any mathematical explanations (namely formulas) is because I'm pretty sure some wiseass already wrote post about it, above mine, or is going to do it in a bit.