"Why is this correct?" "...It just is."

Noah Lukeman's "The First Five Pages: A Writer's Guide to Staying Out of the Rejection Pile" is a straightforward guide to avoiding bad writing. The book is slightly bland, yet it is essential reading material for anyone who aspires to write and write well. Each chapter deals with a particular element of writing, and most of these elements are technical (i.e. Adjectives and Adverbs, Style, Sound, and Comparison).
While reading the second chapter, Adjectives and Adverbs, it hit me: if I were given a formal test on the rules of writing, I would probably do poorly. Grammar rules exist, or seem to exist in my mind as something seemingly innate. When a sentence or paragraph is grammatically sound I do not know this because of a definition, I know because it looks or sounds incorrect. Perhaps this has to do with the fact that I am not an English major; maybe only those who are instructed in the lingo of writing actually understand why the elements of a sentence perform specific functions.
My reaction to reading the definition of the semicolon is a glaring example of my inability to describe the technicalities of sentence construction. This particular definition is a couple of lines long. I can understand it, but could never have come up with it myself. If, prior to reading this passage, someone had asked me how a semicolon is used, I would have been at a loss for words. However, if that same person had given me a paragraph with missing semicolons and had asked me to insert them where appropriate, I would have had no problem.
I am curious to see whether or not anyone else perceives language and its rules in a similar way.