The environment we all live in generally has too many stimuli in it for anyone to process all of them. That simple. Fortunately, we don't have to process all of them to survive. If we miss a fly, we're probably OK. Miss a truck speeding down the highway. not good if we're standing in the highway, maybe.
You can see when we ask people to describe each other on the Internet that we got different descriptions of the same person when we had no idea what they looked like. No reason for them to be the same. Where did they come from? There's really no way to answer that question in general.
A really fascinating piece of research was done at Dartmouth in 1960 around the Presidential Debates. There were four, one was radio only, three were televised. Dartmouth took there Debating Society and only allowed them to hear the debates. Their job was to determine who won the debates. Then stratified samples were constructed, so that there were both Republicans and Democrats in each group, and one group watched the debates on TV, and the other listened to the debates on radio only.
After the debates were over, and the groups were all interviewed and the results tabulated, the researchers were astounded. The Debating Society had clearly scored Nixon the winner of the debates. So had the group who had only heard the debates on the radio. But the group who watched the last three debates on TV gave the debates to Kennedy. The election in November of 1960 also went to Kennedy.
Nixon had looked haggard, tired, he had been on the campaign trail. Kennedy looked like he had just come from a Bermuda vacation. He was young and tanned with a wonderful smile.
Subsequent research has shown that in a visual situation, tonality accounts for approximately 55% of what is 'heard', appearances account for about 30%, and the actual message itself, the so called content may rate as high as 15%.
I don't suppose that anyone here ever thought that someone wasn't listening when they were talking to them recently? Or that people just weren't getting what you were talking or writing about?