Reading in silence

I would like to ask why I am able to concentrate reading a book when there is complete silence, or as well when it is the complete loud chaos - but not in situations inbetween. I managed to isolate my self from the outside world in a constipated train where crying babies and discussing students were evaporating. I managed as well to ignore what was happening in the striking airport sitting on the floor or a busy waiting spot. Or, I was reading listening to the silence of the living room in a winter evening, when no cars drive on the iced road and no birds fly.

Still, I wonder why I am not able to concentrate when it is not complete silence, but not extreme loud as well. I discovered myself listening to the ticking of the rain on my window, distracted from what I was doing and keeping distracting myself. Or, listening to the conversation on the tram of two ladies behind me, talking quietly of boring arguments like what to prepare for dinner. Boring enough, but still not allowing me to concentrate.
Maybe it just depends on the willingness to concentrate. In an office, there is always some kind of noise, but still I have to do some work, and ignore what colleagues say on the phone with someone else or to each other. I mostly manage, unless they say some magical words like my name, or "lunch time".
Our brain is so amazing. So selective, so able to be steered by willingness, by external situations, by the environment. And at the same time, so sensitive to changes, factors, which are allowing us to concentrate on something and still be aware of what happens around us in an alert-enough way.