I think it's because as we practice we are basically trying to 'crack the code' of a piece.
As with a magic eye picture, some can see the image immediately, some it takes a moment, and some never seem to get it. The ones who see it right away say it's a snap and can't believe the others don't get it, whereas the ones that don't get it, don't even believe there is a picture to be seen :D Yet, everyone has eyes and unless there is some vision problem, should be equal, but they are not, and it's all to do with perception.
Music is the same.
Sure some music is technically difficult, but it's surprising how rarely that's actually the case once you reach a certain level. It's more figuring out the code. That doesn't mean you practice less, it's that you really dissect the piece, practice little sections at a time and do whatever it takes for you to crack it.
Back to the magic eye; The ones in the middle (most of us) squint, turn our heads, move closer, move back, cover one eye, ask for hints :D, all kinds of things to crack it...it's the same with music...and the best part of all, is once you have cracked one type, the next song in that style or genre is that much easier as you have the code partially cracked already.
That's why I post up and do lots of different genres; more codes.
One of the best examples of this is the Pink Panther. It's a very easy song, once you get past all the accidentals and unusual chromatic movement. Some say it's really easy, some say it's really hard...but I think it's just a 'code' we don't see often...like magic eye, some see it, some don't.
