Unexpected Encounter

I was sitting at a cafe with my dad when I heard someone say, "Excuse me." I turned and saw a young, somewhat disheveled man standing next to our table. He was looking at me. He then added, "Is your name Denitza?" It was hard to hide the look of surprise on my face. I had thought that he would ask for money, try to sell us something, or something of the sort. After confirming that my name is indeed Denitza, he waited for a few seconds. I guessed that he was waiting for me to recognize him, but I really had no recollection whatsoever. Instead I apologized that he did not look familiar. He said, "We sat at the same desk in 4th grade." I was embarrassed. Normally, I am really good with both faces and names... but I had no recollection that I had sat next to a boy in school, let alone what his name was. Turned out it was Anatol. I slowly started remembering things. But he looked so different to me. Plus, I had only been at that school a year and 1 month. That was my excuse to myself (a poor one, I agree). My dad invited him to sit with us for a bit, which he did. He proceeded to tell us how he had recognized me right away, at which I laughed. Guess I haven't changed at all since I was 10? It was strange to be on uneven ground though - here he was knowing full well who I was, whereas to me he was still a complete stranger.

How is it that some things we remember full well whereas others we seem to have erased completely from our memory?