Preconceptions

Several people asked me to write a short analysis of my winning short-story - a few lines explaining my thought processes and techniques. I duly obliged, and the response to this was surprising.
When I revealed my techniques for creating anticipation - for controlling the reader - several readers were outraged that I had so successfully gone into their heads and manipulated them.
I spent some time thereafter considering my responsibilities.
However, to deny the writer's requirement to manipulate the reader seems, to me, rather naive.
Where is the story formed?
Not on the pages of a book. That's just a bunch of words.
The magic happens in the reader's head, where those words are mingled with the reader's unique experiences and beliefs and thoughts, and are interpreted accordingly. We throw words at the reader, and those words go into the reader's head, along with everything else in there, and the reader makes a story. We, as writers, do no more than manipulate or, if you prefer, shepherd the reader.
Consider this line:
She is a young girl.
Picture her.
We will all create our unique young girl.
I became particularly aware of this when I considered two books I had just finished reading. One was Pullman's Northern Lights. I forget what the other was. And in my mind, Lyra and the second young girl protagonist looked identical. That made me think!
I kind of liken this to Plato's world of ideas. We all have our template of a young girl.
The writer is going in at a point where that young girl exists, and must then refine the image - all of these unique images - and gradually mould the template into something that they/the story require/s.
I'd be very interested to discover where this template originates. Experiments suggest that this girl is not a single person, but is an amalgamation of family members and friends. However, I may be off course here.
I'm also interested in what I can rely on. Do I need to explain that she has two arms? Or, what will happen if I later explain that she has one arm? If the reader has automagically given her two arms, I'm liable to upset them.