Good point about the impact of symbols interacting in any spread. I'm learning - and it's like the more I learn, the less I 'know' - so many layers to a good tarot reading. It's like learning a language and context is everything. A single card takes on a whole different meaning depending on the cards surrounding it.
I think there's also a personal element to interpretation - what the symbol means to both the querent and the reader. The more I talk to long time readers, I find they've created their own methods.
I guess I'd compare it to being a doctor for 30 years vs. a student fresh out of med school. After years of experience, a doctor may be able to diagnose a rare disease almost instantly from one or two symptoms he's seen in other patients over the years, while the new doc wouldn't have that ability/intuition and resorts to the book so to speak.
Definitely right about the personal element @blunderbabe, this is some thing that I discuss in my book. Developing an understanding of how your intuition plays with the symbols, selecting the important etc., and learning the High Priestess' lesson is key to developing a more complete understanding of Tarot.