Anthony Burgess once said that the dream seldom survives the first paragraph.
And Edith Wharton, quoting an old French proverb, wrote, "I dream of an eagle, I give birth to a hummingbird."
All writing is about that gap—no, that chasm—between expectation and final product. A veritable Grand Canyon. It is the very thing that induces writer's block.
I say that we writers should just expect that disappointment, not be surprised by it. Expect it, sidestep it, move on.