I get that ISFP can be quite variable depending on several factors (general intelligence, family history, etc.), but I'm wondering why that is. Fi? Well, at any rate, it's interesting to hear from people who experience the world through SP functions when I can only do as such through artistic experiences.
My best friend growing up was an ISFP. He was basically the reason that I didn't get picked on as a kid. He was really tough, and early on in Kindergarten he decided that we were best friends. I didn't get much say in the matter, but I wasn't inclined to protest very much. I'd describe him as very interested in cars, physical activities and mischief of all sorts. When he discovered girls, for a few years he was basically a walking hard-on. Eventually, he fell in love with a strong-willed woman and got roped into her life. After that, I hardly ever heard from him again. Last I knew, he'd put on a lot of weight, accepted his wife's brand of Jesus, and was a father of two.
My wife is also an ISFP. She's very different from my old best friend. She's quite into the fine arts. She's a graphic artist, painter and a musician. She's very interested in computers although she would probably never frequent an Internet forum unless it was useful in answering specific questions she had about music, art, technical computer stuff or musical instruments. We have a few interests in common - computers, music, going to the gym, etc. Our relationship basically works because we have fun doing stuff together, and we're both pretty willing to be tolerant of our differences.
ISFPs and INTPs fundamentally approach the world in very different ways. I've had 30+ years of being very close with people of this type. I can't really claim to understand what is common to their viewpoint as a whole, but I have a rough feel for how it's different from my own, and I respect it.