Why does a memory change over our lifetime?

It truly is interesting that we can be certain we remember things correctly while actually being completely wrong. There have been studies that show that memories change because they become malleable each time we re-remember them. When we first remember something, synaptic changes known as "consolidation" occur in the brain. When we revisit a memory, that memory has to be reconsolidated once again in our brains. When that happens, the memory becomes vulnerable to new information that was not part of the original memory. Then, when we recall the memory again, it may have elements in it that were introduced from the previous recollection.
This happens because our memories are not simple, straightforward records of our experiences. Instead, it's more like a game of telephone that you're playing within your own brain. Each time a memory is recalled, it can change just a little, and over time, it might become increasingly unreliable.
https://news.northwestern.edu/stories/2012/09/your-memory-is-like-the-telephone-game

https://www.healthguidance.org/entry/17590/1/how-your-memory-changes-every-time-you-access-it.html