People remember highlights and piece together memory later. If you ask when the two cars hit vs when the two cars smashed together people will give you a much more horrific description.
If we remember every mundane piece of information we would likely go nuts from boredom. (I know I myself don't really care what I ate for breakfast ten years go on an exact date.

Sensory Memory lasts for about 0.5 sec for visual and 2-4 sec for audio.
Short Term memory only last about 20-30 seconds without rehearsal.
Long Term memory is permanent unless damage takes place.
Everything about memory is not yet known.