Memories are the stories that we tell ourselves about the past. Sometimes they adjust and leave things out.
In an interesting passage in Think: Why You Should Question Everything, Guy P. Harrison talks about the fallibility of memory.
Did you know that you can't trust even your most precious memories?
They may come to you in great detail and feel 100 percent accurate, but it doesn't matter. They easily could be partial or total lies that your brain is telling you. Really, the personal past that your brain is supposed to be keeping safe for you is not what you think it is. Your memories are pieces and batches of information that your brain cobbles together and serves up to you, not to present the past as accurately as possible, but to provide you with information that you will likely find to be useful in the present. Functional value, not accuracy, is the priority. Your brain is like some power-crazed CIA desk jockey who feeds you memories on a need-to-know basis only. Daniel Schacter, a Harvard memory researcher, says that when the brain remembers, it does so in a way that is similar to how an archaeologist reconstructs a past scene relying on an artifact here, an artifact there. The end result might be informative and useful, but don't expect it to be perfect. This is important because those who don't know anything about how memory works already have one foot in fantasyland. Most people believe that our memory operates in a way that is similar to a video camera. They think that the sights, sounds, and feelings of our experiences are recorded on something like a hard drive in their heads. Totally wrong. When you remember your past, you don't get to watch an accurately recorded replay.
To describe to people how memory really works, Harrison puts it this way:
Imagine a very tiny old man sitting by a very tiny campfire somewhere inside your head. He's wearing a worn and raggedy hat and has a long, scruffy, gray beard. He looks a lot like one of those old California gold prospectors from the 1800s. He can be grumpy and uncooperative at times, but he's the keeper of your memories and you are stuck with him. When you want or need to remember something from your past, you have to go through the old codger. Let's say you want to recall that time when you scored the winning goal in a middle-school soccer match. You have to tap the old coot on the shoulder and ask him to tell you about it. He usually responds with something. But he doesn't read from a faithfully recorded transcript, doesn't review a comprehensive photo archive to create an accurate timeline, and doesn't double-check his facts before speaking. He definitely doesn't play a video recording of the game for you. Typically, he just launches into a tale about your glorious goal that won the big game. He throws up some images for you, so it's kind of like a lecture or slideshow. Nice and useful, perhaps, but definitely not reliable